I would get some great Musicans & Make Great Levels with Gimmicks (Like Dodging Bullet Bills that come from the Background) The Final Boss would be Bowser in the Backround with you using the Gold Flower To Hurt him. the Difficulty would also be Pumped.
They should have put it some store mechanic like in some SMW romhacks
Before they get into stuff like that, they need to put in new graphics, music, design some new original characters, enemies and put in some new original mechanics to use in levels.
This is precisely why SMW hacks always horrendously suck; they focus on the irrevelant stuff first, like "oou, we'll have a shop", instead of making their terrible, claustrophobic levels actually fun to play, and polishing up the mechanics they use in levels.
So, really, don't say "hey, put this something from a SMW hack!", because they really aren't the standard Nintendo should aspire to achieve.