Ok, I'e played the available levels of your beta, I'm going to give some feedback.
The first, and the thing I most liked in your hack is the level design: every level is very well-planned and it is fun to play.
The graphics choice is good as well, every level have its style, and I really liked it.
I loved the music choice as well, just AMAZING!
The only thing that is lacking in your hack are different gimmicks, I'll explain: all the levels suppose Mario to just go through the level without any particular gimmick. It is important to vary the gimmick in some levels, or it'll be boring. Some examples could be, using particular blocks (for example Ice Blocks that can be destroyed with fireballs, wall-jump blocks...), different sprites (the YI numbered platforms....) or even better, ASM changes and/or settings. In short, just make Mario interact with different stuff/sprites/etc. in different ways in each different level.
Remember: your level design is very good so far, but you must implement some new gimmicks in your levels to not make them too boring for the player.
Keep up the good work, you'll be doing a great job!