What about a hack of Ocarina of Time where you could enable/disable the Iron Boots with the press of a button?
Ala 3DS version.
That would greatly enhance the experience of the game, bringing down a well known nuance!
One which puts the two types of boots as C button items would be even better (and just like the 3DS game).
On another note, you know what I'd like to see? ROM hacking related apps for iOS and Android phones. It'd be a great way to get more people into this hobby, and could even mean we'd get tools with decent interfaces and what not (instead of a bunch of command line stuff and really archaic looking Java programs).
Even better if you could draw the level tiles like in Smash Bros for Wii U's level editor...
Oh, and here's another idea. How about a Super Mario Bros 3 crossover ROM hack? Cause for some reason, almost all crossover games seem to be SNES styled. So it'd be nice to see a SMB 3 hack where Mario fights Dr Wily, Dracula, Ganon and other popular eight bit era villains while exploring levels based on Mega Man, Zelda, Castlevania, Ghosts and Goblins, Ninja Gaiden, etc.