Well, first thing I must say, I really liked how the levels of this hack don't use so much different sprites. Most of the standard hacks I played repeated the same enemies without much creativity, like tons of volcano lotus, chargin chucks and so on. So yeah, most of the time, the enemy variety of this hack is quite good.
Anyway, let's go to some points that need to be fixed.
Intro Level:
I don't understand what are you saying. Do you mean " Welcome! While Luigi was at Neo Green Plains, Dark Luigi kidnapped Mario. Let's save him, Luigi!"? If yes, then I suggest to rephrase it, since this sentence is a bit confusing, althought it's very easy to understand that instead of Peach, it's Mario who got kidnapped. So, this is a minor thing.
HDMA in some stages, in general:
I suggest you use more lines and make a more varied gradient. It's not bad, but I think it can be improved.
I suggest you change the name of each level. Why every level is the name of the world? Would give them more flavor. Let's say that 1-1 is " Green Path" and 1-2 is like "Verdant Hills". See? Much better and the player will remember well the stages. It's just some examples. Don't use these names, what I'm saying is that if you're using the patch of YI intro for levels, it would be best that each level had a diferent intro, rather than "Neo Green Plains".
First Mushroom Level:
I almost didn't see this platform, I suggest moving it a little bit. It's almost a blind jump.
I suggest fixing this glitch by using
this patch. Be aware that it will glitch the credits sequence from the vanilla game. So, only use that fix if you will not use the vanilla ending, otherwise, it's better to wait for the remoderation of this patch. This happen in later levels, too.
Night Plain Level:
In my humble opinion, Sound of the Shinobi does not fit calm night levels so well. I would suggest a more chill music. Anyway, just a suggestion, this is not necessarily an error.
While most of the blind jumps are harmless, this one isn't. I died trying to get the coin. Please avoid that. I'll talk about blind jumps in the end, so hang for a while.
I think this jump is a bit too demanding for a first world level. i suggest to nerf it or tone it down. Make it a bit more easy.
Neo Green Castle:
The positioning of this Thwomp is not that good. I got caught by surprise, since the top hides part of the sprite. I suggest you lower it a bit, so the player is not caught by surprise.
Samba Fortress:
For the sake of consistency, this door should be red like the others that I saw in the end of the castles and fortresses. It looks like a normal door. It's very easy to fix.
First Desert Level:
The palette of the foreground is bizarre. While the background uses a more calm color, the FG looks like cheese with that yellow tone. I suggest you change it to match the background, while using a brighter and more vibrant tone so both doesn't get mixed up. Try using some hue shifting to solve this.
Another blind jump that can potentially kill the player...
Desert Cave level:
Not only it's a blind jump, but I got lucky that the beach koopa didn't hit me with the shell. I found the positioning of this enemy kinda cheap and it could take the life of a less careful player very easily. It's not fun.
Desert Canyon Level:
The music is way too repetitive and gets annoying way too fast. I think you should replace it. It's like a ten or fifteen second loop that grows old very quickly.Also, I'm not a big fan of this slow autoscroll with bullets, it kinda tires the player a little bit, but anyway, my major gripe is the music itself. The autoscroll part I handled fine, though.
Desert Fortress:
Another case of boss door not being red. Like I said, it needs to be consistent. It's an easy fix.
Angry Sun Level:
How I get that coin? With that sun following me constantly, I didn't explore the stage very well? It's a P-switch? Or a spin jump in the sun? It's not so intuitive...
Night Desert Level:
Same things as I mentioned about the music not matching well the level that I said about the Neo Green Plains night level.
Desert Castle:
Another blind jump that punishes the player with death. Only because of a coin. Please add some indication like coins. It's hard to guess sometimes when it's safe to fall or not.
For some odd reason, the desert castle didn't collapsed after beating it.
Mountain Fortress (and other levels that use this graphic):
How I was supposed to know that these are SPIKES? They don't look sharp enough. I suggest replacing these spikes for ones that are pointy.
Night Mountain Level:
Once again, I think the music does not fit. Why a music with a war-like vibe (Dark World from A Link to the Past) in a very chill night level? Not gamebreaking, but it kinda doesn't fit well with the level, in my opinion.
First Forest level:
Change the scrolling of this stage to "No Vertical Scroll" so this cutoff doesn't appear.
Second Forest Level:
For some reason, I start the level already in the midway.
This block was kinda cheap and kaizo like. I suggest to change its position, since it caused one cheap death.
Forest Tides Level:
How I was supposed to know that the water is poison? The water should have a darker purple than the foreground. Add a danger sign or something in the level.
I found the positioning of this enemy a bit nasty. It's possible to pass this part, but requires a precise spin jump.
Forest Fortress:
I think this section is way too tedious because all you have to do is wait for the magikoopa to destroy the block walls. Nothing too much interesting happen during that second half. I think the pacing of this second half suffers quite a bit.
Why this boss door has no powerup block? Doesn't make any sense...
Forest Cave:
It's possible to cripple through the ceiling in this section. Be careful about that, since it can cause some unfair deaths.
Autumn Forest:
Another stage that the player starts in the midway point.
Night Forest:
This koopa always try to kill itself. I suggest you apply
this patch. So any enemy will always face Mario and it will avoid situations like that.
Forest Castle:
This ON-OFF setups were okay in the first, in the second time. But a third time? I would suggest a new gimmick for this castle, since it's a bit repetitive the on-off stuff again. And that's what I think that some levels need. Not only varied enemies, but some interesting gimmicks to be more memorable. Anyway, this is a minor critic I have.
Change the color of the fire spitting fuzzies. I have trouble to differentiate them from the normal ones. Only the different color of the mouth won't do. You can use the cfg editor (included with PIXI) to change the palette of the sprite.
Change the color of the lines, I can barely see them.
BLIND JUMPS:
These are the moments I found blind jumps. I suggest adding coins or avoid blind jumps if you can. While most of them are harmless, it's kinda frustrating when I can't see the ground below.
Overall, I see tons of potential in this hack. Keep going forward!