Tip: By pressing Shift while placing event tiles, they will be made "silent" and will not appear until a map transition happens. Always use silent event tiles to draw paths in other maps (i.e. a Vanilla Dome event while the player is on the main map), or glitchy tiles may appear.
IGNORE Some parts will probably have to be changed around (the area between the first coin and the normal/secret split, perhaps), and I'm not too happy about the area containing the third coin, but...
Also the pipe leading to the secret exit currently leads nowhere. Just letting you know that.
EDIT: somehow I managed to choose the wrong file. Guess that's what happens when you do stuff when you're pretty much exhausted. Here.
-The difficulty is slanted a bit too much toward both the former half and lower sections of the level.
-Some sprites are pretty close to the ceiling generator, and since the ceiling's invisible, players' jumps might be unfairly cut short, leading to hits that seem cheap.
-Piranha plants only disappear at the start of the next emergence, not at the end of the previous, which makes some parts- A, upper, for example- more annoying than they need to be.
1: This would be a nice introduction if the quicksand were a bit more visible, as opposed to only lining the bottom row of the screen.
2: You can get this smwcoin from below.
6, lower: I think that piranha plant is too close to the coin guide. What if you separated them a bit more?
A, upper: Apart from the aforementioned issue, that fire bro is very hard to defeat thanks to the ceiling generator, and its fireballs can follow you into the next piece of quicksand...taken together, it seems a bit unfair.
E: Something about the way both the venus trap and fire bro are both all the way to the left here makes this setup quite difficult. I'm thinking you should move one of the sprites back a little and/or shift the quicksand back a bit and/or have it slope down a bit into the quicksand instead of level terrain.
F: This midpoint feels a bit late, especially since the second half of the level is easier, and its placement also means there's no midpoint for the secret path, unless you include another one just for that purpose.
14, middle: I don't see why anyone would want to drop down to the bottom section here.
16, under smwcoin: There's a ledge tile here that should be a cement block (in terms of what it acts like), and you can get the smwcoin from below.
19: Thanks to the oh-so-fantastic "Yay physics enabled!" goal roulette sprite, you can die if you jump into the quicksand. Honestly, this is getting absurd- I like that patch of quicksand where it is, and I don't see why we have to surround the goal point with four screens of boring nothingness just to appease a mechanic that doesn't do anything worthwhile for us in the first place. I'm going to bring this up in general discussion again.
Overall, I like where this is going, but it does need some polishing (and a secret path, of course) before it can be readied for clearance.
Version 2. Now with the secret path.
The main path still needs a fair bit of fine-tuning, but I did mess around with it a bit, putting in a few of AxemJinx's suggestions. Sadly I got a bit lazy and just tried putting in a few enemies to balance the difficulty of the second half - how that went is debatable.
[I might actually scrap a few sections of the post-midpoint bit, but not right now.]
Overall, it looks pretty good. I do have some comments, though.
2, lower left: Unless players know Mario has a lower fall speed when holding the jump button, they'll probably take the top path here, and overall the jumps involved just seem unnecessarily tricky for this early in the level.
3, top: Personally, I would lower this koopa a bit, since at its highest it's basically at the ceiling generator.
7/8, top: This koopa can get oddly stuck in the dirt if hit at the right time. I would suggest moving a tile, maybe back a couple of tiles too.
8, middle: I wonder whether there should be an enemy on this ledge? I'm not sure.
6/7, bottom: Because of that piranha plant disappearing behavior stuff I was talking about, this setup does seem a bit difficult...I suppose it's nullified by the powerup immediately after, though *shrug*. So it might be fine- see what others think, too.
D, bottom: Personally, I would raise or lower the ceiling above those blocks by a tile.
10-12: I don't know how much more sprite room you have here, but I keep feeling like it could benefit from one more sprite. For example, instead of or in addition to the kicking koopa, maybe a boomerang bro or something behind that first quicksand pit? Also, for the level in general, I feel like you're missing out just a bit on the opportunity to place enemies behind quicksand pits interspersed with steep slopes out in the open like you do on these screens. Personally, I'd like to see an even steeper version of this (ascending or descending or both) at the end of the stage.
16-17, top: I think the p-switch gives you more time than you'd ever need; maybe an on/off switch would be more appropriate?
16, bottom: (see 6/7, bottom)
2, top: Since you can jump up here, I feel like there should be an enemy, even if it's a simple one, considering the bottom path has 3 enemies total.
7, bottom: To make the lower path here more feasible, I think you should widen the opening to the tunnel by a tile or two, probably on the top so the kicked shell isn't affected.
B: The jumps down to the midair platforms seem slightly blind, more the first one than the second. I'm wondering if you should at least raise the first one by a tile.