It's pretty impressive for a first hack, quite good with some minor issues.
Mario's House: You just need one message box in that level, apart from the Save Block. Messages kind of mess up:
This message can also be read in Keelhaull Beach 3:
Mushroom Meadows 1: A coin indicator would be a good idea to know that the wall is fake, also, I suggest making the upper part solid, as Mario can go up and can't go through wall on the other side:
I think increasing the timer a bit may be good, just a suggestion.
In the oveworld there isn't a yellow dot for Mushroom Meadows 2:
Mushroom Meadows 3: Cutoff in the sublevel where the secret exit is:
Mushroom Meadows A:
The indicator is there but, the same issue where Mario can go up is still there:
Also, Big Mario can't pass easily between the Munchers and the small pipes:
Mushroom Meadows 4:
The secret passage works great here, keep it in mind for the passages in Mushroom Meadows 1 and Mushroom Meadows A:
The 4th Peach Coin didn't appear on my first run, but it did in the second:
Minor cutoff between the rope and the foreground, also between the lava and foreground, but don't mind about that one:
Mushroom Castle: Cutoff between the castle blocks and the foreground, also a wrong corner tile between the foreground where Mario is standing and the semi-solid ground:
Keelhaul Beach 1: The location of the 2nd Peach Coin is kind of weird, cause there's a blue pipe next to the checkpoint which takes Mario to the underwater sublevel but the coin is kind of far from the checkpoint and you have to return back there after getting it. Apart from that you can skip the underwater section by swimming under the level:
Here it would be a good idea to increase the timer because of that.
Keelhaul Beach 2: It's possible to enter the upper pipe to get the secret exit, apart from the first one:
Also, cutoff between the clouds and the pipe:
K. Koopa's B. Ship: Mario can't get the 5th Peach Coin in a normal way. You have to do a tricky swim under the ship to get it. You might want to change the coin location or shorten one or two tiles that last ship where the pipe is:
Keelhaul Beach 3:
You can swim under the level as a shortcut:
The 5th Peach Coin glitches because it's located in a subscreen boundary and you can farm some lives because of that. You can increase or decrease the coin position by putting it one tile up or lowering the coin 1 tile:
Keelhaul Beach 4:
Big Mario can't get the Fire Flower unless he does a precise duck jump:
Also, there are only 4 Peach Coins.
There are some doors that can't be accesed, that one for example:
If you enter the door in the upper part of the screenshot (almost can't be seen), you're supposed to get the 4th Peach Coin, but after entering it, the coin's not there for some reason:
This is the most unfair part of the hack, Mario has to get hurt by the Urchin to get the 5th Peach Coin and have a reserve Mushroom because Mario has to be hurt again to come back:
The battle against Clawgrip uses the SMAS: SMB3 - Castle Music instead of a boss music and the song finishes while fighting against the boss:
Most of the issues are easy to fix. Apart from that, the hack is quite good! Looking forward to a next demo!