I have an idea for a level, which I think would be awesome.
A factory level, with the song "Breakfast Machine" from Pee Wee's Big Adventure playing.
I would suggest that the level be styled a la The Blue Ball Machine, with crazy synchronized factory stuff happening everywhere involving platforms, enemies, and probably lots of custom graphics and events.
I don't hack so I do not know the limits and boundaries for a level hack such as this, but I believe that, if executed correctly, this kind of level would be epic.
My idea: Like Frankenstein's Monster~ this... thing, its comprised of pieces/body parts of all the previously defeated bosses.
I have not been following the storyline of this hack, but I take it that Doc Croc is the big bad guy of SMWCP2.
So mario comes in to discover Doc Croc is back, perhaps all beat up and bruised from his previous encounter with Mario, and he is just implementing the finishing touches to his derranged masterpiece in an attempt to re-animate the assorted remnants that he managed to salvage from the previously defeated bosses (his previously failed expiriments? Like I said I have not been following the story )
Anyway this thing comes to life and it either (a) follows Doc Croc's commands or (b) Demonstrates its power, and the folly in Doc Croc's plan to play God by swiftly gobling him up or swiping him to the side or something like that. At this point the player should not really know what to expect from this fight except that it is going to be ridiculous and that this creature was never meant to be alive, it is very powerful, unpredictable and very pissed off.
It is one of the greatest rom hacks ever created. It features the same save system that you are working on, and everything is interconnected: there are 5 "areas" that you can go to and from, and which each have custom graphics/enemies/music. It features a number of power-ups that you can find, including screw attack and space jump.
If this hack is actually to be like Adventure Island at all, what made that game unique was that when you collected fruit you gained time. There was not enough time on the clock to finish a level without collecting fruit. This made the game interesting by encouraging the player to move a little faster and to make the player tackle obstacles that may have been avoided if not for the rewarding fruit.
So I'm just putting it out there that a good gimmick for an "Adventure Island" styled hack would be the same kind of time limit behaviour.