this is very cool! I 100% the entire game and had lots of fun doing so. but there are some things that I feel could be improved or added:
the amount of lag the camera has vertically in 2D mode can be a bit disorienting, and can also lead to jumps being a bit hard to do since it takes a lot longer for them to get in view.
another thing that bothers me a little is also the apparent lack of jump buffering (as in the game keeping your jump input for a while if you press it while in a state that you can't jump in, and then executing it once you're able to jump), it leads to it feeling like the game consumes your jump at times as it only registers a jump input when you're on the ground.
there's also something that isn't really important but a bit weird looking, that being the inconsistency with fonts. the slash in the crate HUD is seemingly from some standard font instead of the crash font, and the "BONUS" text in bonus stages also uses an HD version of the font, unlike the rest of the HUD. it's not at all important but it does lead to the HUD looking a bit funky.
the last thing I want to comment on is the lack of the slide spin, it's not super important but it does allow for some more movement tech and does make spinning while sliding feel a lot better than it does right now.
overall though this is a very cool and pretty impressive project and I'm excited to see where it'll go from here.