Some ideas I thought would be interesting to see in a SNES ROM hack (which aren't going to be in my own project):
1. A near complete remake of Castlevania Symphony of the Night for the SNES. Yeah it'd require quite a few compromises on animation colours and boss battle designs, but you could probably get fairly close to the real game even on 16 bit hardware. After all, I've ripped stuff like the whole clocktower FG/BG sets for my SMW mod, so someone ripping every other tileset and graphic doesn't seem too far fetched...
2. Metal Slug elements, like the Rebel Army enemies. I'd love to see someone try and put these guys in a SNES game/hack of some kind, especially Super Mario World. After all, they worked quite well in Mushroom Kingdom Fusion and Super Mario Fusion Revival:https://www.youtube.com/watch?v=Ns9s6yMHSvg
3. A Mario & Luigi style giant battle system, even as a tech demo. I'd love to see something sort of inspired by this:https://www.youtube.com/watch?v=DyYwkYF8y3g
Except with 16 bit sprites and music, would be just awesome to see running on a SNES.
4. Wario Land 4 for SNES. I love this game, and damn I'd love to play a version of it for the SNES. Tech wise it would need a bit of toning down (especially layer count and level scrolling wise), but it wouldn't be completely impossible, at least from my experiments with Wario Land 4 style levels in Super Mario World's engine.
5. Super Mario Country as a Game Boy game. This Mario fan game looks really awesome and plays pretty well, and it's Super Mario Land 2 inspired game design would be perfect for an actual ROM hack or homebrew title:http://mfgg.net/index.php?act=resdb¶m=02&c=2&id=30256
those are some random ideas I had for ROM hacking and stuff. They're not ones I'd ever attempt myself (though elements from the above ideas can be found in my own projects), but they're definitely ones I'd love to see someone else try sometime in the far future.