Hack Name: Bowser Stadium
Author: levelengine
Description: Mario winds up on an island of Bowser's where he must compete against Bowser's 19 new courses.
The game is cleared when you beat Bowser, and to do so, you play all the other levels until you find 4 different switches to form the bridge to Bowser's castle, similar to EVW.
New tricks:
-Two-toning and new tiles (a spiky star in a castle works like a normal spike)
-Toxic water (you could only survive for 30 seconds)
-Evacuation (blowing up a reactor core then getting the hell out)
-Difficulty levels (explained in Introduction #2)
-3-Up Moons hidden in every level with a medium difficulty or higher.
-Variety of ways to finish the game.
-Major reduce of unfair situations, flaws, glitches, and failure to load sprites
General issues
I must admit this isn't an ordinary hack. At first I was surprised I was able to access every level on the overworld, but later I understood the mecanic of your hack. Maybe you could make it a bit more challenging, like having a normal exit i every level and a secret one that leads to the switch. This is just a random thought of mine though, so feel free to ignore it.
Most of the levels had a good design; unfortunately there were also a few levels that were poorly designed. Moving on to the issues:

Not a removal reason, but the title screen was really boring. Also the file selection menus has a bad palette.

This switch looks golden, but when I press it, it turn blue, this happened with all of your switches, use a custom palette and change to sprite color to fix it. Also, all the switches give me the same "you found a golden switch" message regardless of their color, and they all spawn yellow blocks on the overworld.

It's kind of weird to see Mario walking against the wall, you know.
--
Overworld issues

Minor, but this level could use an actual tile (Yoshi's House maybe?)

There are some cutoffs on the border, there are some tiles in the original SMW for OW borders with paths, why don't you use them?

What's with the pipe in the middle of the water?
--
Introduction #2

I'm sure you could have put this as a second message box in introduction #1
-Not pictured-
Overall this level's design was poor. It was very empty with almost no ennemies around; I know it's supposed to be easy, but easy doesn't mean empty and boring. If you want to make an easy level, think about something like the fisrt world of the original SMW.
--
Koopa peninsula
-Not pictured-
Once again, this level had a poor, empty and kind of repetitive.

The camera started a bit out low here. Check the F.A.Q. for info on how to set the camera correctly. This also happened in a few other levels, by the way.

Here I died because I thought the blue frame at the bottom was water. Place the background correctly.

It's kind of hard to get that Yoshi Coin without getting hurt.
--
Freaky Fantasy
-Not pictured-
The first segment of this level was really boring and empty.
--
Muncher Fears

Minor, but the background doesn't look good. Also, the cement blocks cutoff the bones.
--
Forest of pipes

The cement blocks cutoff the pipes.

This is kind of annoying with big Mario. I advise you to avoid these kind of situations in general.

This is where I ended up after taking a Yoshi Coin. Don't use these kind of traps.
--
Iceberg Passage

Cuttof between the foreground and the dirt because of bad corner tile.
--
Frenzy Plant

Don't use the bottom row of Lunar Magic, it doesn't appear in game.

Cutoffs bellow the slope.

The cement blocks cutof the FG.
--
Bouncy Trouncy

Bad buzy beetle pallete.
--
Mountain Trudge

This segment was made almost entirely of map16 inserted switch palace blocks. Why don't you use actual FG tiles for this? You can use the map16 editors to make skinny FG tiles. Same goes for other levels with these tiles. Also, there are some glitched BG tiles at the top, you can fix that by changing the BG scroll (boton with th fishes).

If I die after taking this midpoint, I reappear at the beginning of the level.
--
Valley Crash

Bad corner tile bellow the bone.
--
Switch it to me!

I was able to cross the coin path before the P-switch ended, so now I have to wait here...boring. This happened several times durring this level.

The red switch block cutoffs the ground. I think you should use something else to mark your reset pipes.
--
The hill

Bad dino rhino palette

Now I have to bring this P-switch all the way back. I advise you to avoid this kind of situations because most of the players don't like it. Fortunately I don't have to backtrack much, so this isn't a major issue, but I recommend you not to do this.

The lightning here didn't generate fire because of sprite memory issues. Change the sprite memory index (lakitu's head) to 7.

Floating fire never looked good.

The FG block cutoffs the pipe.
--
Haunted outpost
-Not pictured-
This level was way too long and hard compared to the rest of the "hard" levels. You should either decrease the difficulty or label it as "expert".

Great! Now I'm stuck. All this just for a powerup? By the way, this segment was pretty empty and boring.

The cobwebs and the boos don't look good. I know what you were aiming for, but this is not the way. The outline should be darker than the body.

Horrible cutoff between the pipe and the water.

I already read that in the other message box. Also, the pipe cutoffs the water.

The lakitu glitches because of sprite memory issues. Change the sprite memory index to C.

Bob-omb graphics are not a substitute for thwimps, it's not very logic since they don't actually explode. Don't do this.

The tiles below the triangle block don't look good. Use map16 to make a good looking tile that acts like 1EB.

More sprite memory issues. Change the sprite memory index to 7.
--
Dark path

The cement blocks cutoff the dirt.
--
Bullets and Moles

I can easily get stuck here if I don't have a mushroom.

Yeah...I already told you not to do this.
--
Diver Down

Slowdown. Reduce the number of sprites.

The P-switch sometimes refuses to appear because there are too many sprites around. I advise you to remove it, it's pretty pointless.

The jump here is quite hard to perform. I know this level is supposed to be very hard, but it shouldn't requite this kind of precision.

It's pretty easy to kill the disco koopa by accident with the throw block and get stuck here. Add a reset door.
--
Cloudy Heights
Ok, so this level (along with Entry of Bowser #2) is the main reason why I decied to remove this hack. I know the level is supposed to be hard, but this particular level crossed the line: I was unable to beat it without savestates and rewinds since it had a lot of segment that required high levels of precision.
-Not Pictured-
If I kill one of the disco koopas, or any of the enemies I need to proceed I can get stuck. Some of them can be easily killed by accident.

Whee! Let's break the level! Ok… this was the only easy segment of the level, actually.

Really? I have to jump on the exploding block goomba in the middle of the air before it falls down?

This jump is quite hard as big Mario

Just like this segment: I have to duck down in order to proceed, which slows me down so I have no time to reach the safe area and I die.

Ah, come on!
--
Entry of Bowser #1

If I enter the level without having all four switches pressed I'm forced to kill myself, add a side exit to the level (same goes for #2). Also, what's with the scrolling layer 3 rocks background? (not shown in the picture)

The floating spike looks kind of ugly. Sure, it's better than a floating muncher, but I'm sure you can use a better substitute. Also, the lava cutoffs the castle blocks.

This doesn't look like a waterfall, and it's horribly cutoff.
--
Entry of Bowser #2

I stopped here after trying this particular segment (with savestates) over 50 times; you seriously need to cut down the difficulty here. Also, the FG, BG and thwomp palettes look ugly, and the candles lack flames (use sprite E6).
Summary:
Overall, a few levels (most notably Introduction #2 and Koopa Peninsula) need to be improved level design-wise; and a few others (Cloudy Heights and Entry of Bowser #2) need to be cut down on the difficulty. There are a lot of cutoffs around the hack; while they're not major issues, you should probably fix them; try to edit the cement block graphics so you can give them square edges.
Fix the issues I mentioned (especially the ones related to level design) and I'll accept this. Read the Hack submission guidelines. I also advise you to read this and this to improve your level design skills.
Author: levelengine
Description: Mario winds up on an island of Bowser's where he must compete against Bowser's 19 new courses.
The game is cleared when you beat Bowser, and to do so, you play all the other levels until you find 4 different switches to form the bridge to Bowser's castle, similar to EVW.
New tricks:
-Two-toning and new tiles (a spiky star in a castle works like a normal spike)
-Toxic water (you could only survive for 30 seconds)
-Evacuation (blowing up a reactor core then getting the hell out)
-Difficulty levels (explained in Introduction #2)
-3-Up Moons hidden in every level with a medium difficulty or higher.
-Variety of ways to finish the game.
-Major reduce of unfair situations, flaws, glitches, and failure to load sprites
General issues
I must admit this isn't an ordinary hack. At first I was surprised I was able to access every level on the overworld, but later I understood the mecanic of your hack. Maybe you could make it a bit more challenging, like having a normal exit i every level and a secret one that leads to the switch. This is just a random thought of mine though, so feel free to ignore it.
Most of the levels had a good design; unfortunately there were also a few levels that were poorly designed. Moving on to the issues:

Not a removal reason, but the title screen was really boring. Also the file selection menus has a bad palette.



This switch looks golden, but when I press it, it turn blue, this happened with all of your switches, use a custom palette and change to sprite color to fix it. Also, all the switches give me the same "you found a golden switch" message regardless of their color, and they all spawn yellow blocks on the overworld.

It's kind of weird to see Mario walking against the wall, you know.
--
Overworld issues

Minor, but this level could use an actual tile (Yoshi's House maybe?)

There are some cutoffs on the border, there are some tiles in the original SMW for OW borders with paths, why don't you use them?

What's with the pipe in the middle of the water?
--
Introduction #2

I'm sure you could have put this as a second message box in introduction #1
-Not pictured-
Overall this level's design was poor. It was very empty with almost no ennemies around; I know it's supposed to be easy, but easy doesn't mean empty and boring. If you want to make an easy level, think about something like the fisrt world of the original SMW.
--
Koopa peninsula
-Not pictured-
Once again, this level had a poor, empty and kind of repetitive.

The camera started a bit out low here. Check the F.A.Q. for info on how to set the camera correctly. This also happened in a few other levels, by the way.

Here I died because I thought the blue frame at the bottom was water. Place the background correctly.

It's kind of hard to get that Yoshi Coin without getting hurt.
--
Freaky Fantasy
-Not pictured-
The first segment of this level was really boring and empty.
--
Muncher Fears

Minor, but the background doesn't look good. Also, the cement blocks cutoff the bones.
--
Forest of pipes

The cement blocks cutoff the pipes.

This is kind of annoying with big Mario. I advise you to avoid these kind of situations in general.

This is where I ended up after taking a Yoshi Coin. Don't use these kind of traps.
--
Iceberg Passage

Cuttof between the foreground and the dirt because of bad corner tile.
--
Frenzy Plant

Don't use the bottom row of Lunar Magic, it doesn't appear in game.

Cutoffs bellow the slope.

The cement blocks cutof the FG.
--
Bouncy Trouncy

Bad buzy beetle pallete.
--
Mountain Trudge

This segment was made almost entirely of map16 inserted switch palace blocks. Why don't you use actual FG tiles for this? You can use the map16 editors to make skinny FG tiles. Same goes for other levels with these tiles. Also, there are some glitched BG tiles at the top, you can fix that by changing the BG scroll (boton with th fishes).

If I die after taking this midpoint, I reappear at the beginning of the level.
--
Valley Crash

Bad corner tile bellow the bone.
--
Switch it to me!

I was able to cross the coin path before the P-switch ended, so now I have to wait here...boring. This happened several times durring this level.

The red switch block cutoffs the ground. I think you should use something else to mark your reset pipes.
--
The hill

Bad dino rhino palette

Now I have to bring this P-switch all the way back. I advise you to avoid this kind of situations because most of the players don't like it. Fortunately I don't have to backtrack much, so this isn't a major issue, but I recommend you not to do this.

The lightning here didn't generate fire because of sprite memory issues. Change the sprite memory index (lakitu's head) to 7.

Floating fire never looked good.

The FG block cutoffs the pipe.
--
Haunted outpost
-Not pictured-
This level was way too long and hard compared to the rest of the "hard" levels. You should either decrease the difficulty or label it as "expert".

Great! Now I'm stuck. All this just for a powerup? By the way, this segment was pretty empty and boring.

The cobwebs and the boos don't look good. I know what you were aiming for, but this is not the way. The outline should be darker than the body.

Horrible cutoff between the pipe and the water.

I already read that in the other message box. Also, the pipe cutoffs the water.

The lakitu glitches because of sprite memory issues. Change the sprite memory index to C.

Bob-omb graphics are not a substitute for thwimps, it's not very logic since they don't actually explode. Don't do this.

The tiles below the triangle block don't look good. Use map16 to make a good looking tile that acts like 1EB.

More sprite memory issues. Change the sprite memory index to 7.
--
Dark path

The cement blocks cutoff the dirt.
--
Bullets and Moles

I can easily get stuck here if I don't have a mushroom.

Yeah...I already told you not to do this.
--
Diver Down


Slowdown. Reduce the number of sprites.

The P-switch sometimes refuses to appear because there are too many sprites around. I advise you to remove it, it's pretty pointless.

The jump here is quite hard to perform. I know this level is supposed to be very hard, but it shouldn't requite this kind of precision.

It's pretty easy to kill the disco koopa by accident with the throw block and get stuck here. Add a reset door.
--
Cloudy Heights
Ok, so this level (along with Entry of Bowser #2) is the main reason why I decied to remove this hack. I know the level is supposed to be hard, but this particular level crossed the line: I was unable to beat it without savestates and rewinds since it had a lot of segment that required high levels of precision.
-Not Pictured-
If I kill one of the disco koopas, or any of the enemies I need to proceed I can get stuck. Some of them can be easily killed by accident.

Whee! Let's break the level! Ok… this was the only easy segment of the level, actually.

Really? I have to jump on the exploding block goomba in the middle of the air before it falls down?

This jump is quite hard as big Mario

Just like this segment: I have to duck down in order to proceed, which slows me down so I have no time to reach the safe area and I die.


Ah, come on!
--
Entry of Bowser #1

If I enter the level without having all four switches pressed I'm forced to kill myself, add a side exit to the level (same goes for #2). Also, what's with the scrolling layer 3 rocks background? (not shown in the picture)

The floating spike looks kind of ugly. Sure, it's better than a floating muncher, but I'm sure you can use a better substitute. Also, the lava cutoffs the castle blocks.

This doesn't look like a waterfall, and it's horribly cutoff.
--
Entry of Bowser #2

I stopped here after trying this particular segment (with savestates) over 50 times; you seriously need to cut down the difficulty here. Also, the FG, BG and thwomp palettes look ugly, and the candles lack flames (use sprite E6).
Summary:
Overall, a few levels (most notably Introduction #2 and Koopa Peninsula) need to be improved level design-wise; and a few others (Cloudy Heights and Entry of Bowser #2) need to be cut down on the difficulty. There are a lot of cutoffs around the hack; while they're not major issues, you should probably fix them; try to edit the cement block graphics so you can give them square edges.
Fix the issues I mentioned (especially the ones related to level design) and I'll accept this. Read the Hack submission guidelines. I also advise you to read this and this to improve your level design skills.