As I speculated that I might, I have started a Kaizo hack of Yoshi's Island. Currently I plan for it to be one level long, but what a level it will be! Eggs will only be given as needed and since I don't know very many exploitable Yoshi's Island glitches, I don't think it'll be very heavy on the glitch abuse. I might use the "die in lava and land on a ! switch" in a puzzle segment, but it'll have to be near the end because I don't want the music to cut out for too long.
The bad beginning. I died because I did not flutter jump as soon as the level began. You have to flutter jump and get the egg resting on the spikes, bounce off the Shy Guy, hit the cloud to drop a ! switch, and run like hell while avoiding Fuzzies.
You barely have enough time to get there before the ! switch expires.
This is why you must avoid all the Fuzzies. Apparently line guides act weird when combined with the dizzy HDMA, sometimes causing them to fall off, as shown. I guess that ruins any ideas I had of making a really hard non-Kaizo brutal bonus level called "Drunk Athletics."
I am also thinking about another segment that resembles "Poochy Ain't Stupid" but more difficult, as well as a couple back to back rooms featuring Salvo the Slime (inspired by 6-4 from the original), but you only have the right amount of eggs and you can't get any more, so you must bounce eggs strategically.
Savestates will be needed, but you hopefully shouldn't need to use emulator slowdown. This isn't pit.
--------------------Legacy custom music
How am I so creative? I think taking walks might have something to do with it.
Every single level I will ever make in SMM2 will be easier than Ultra Necrozma.