One thing that annoyed me a little about Melee was that CPU characters weren't affected by visual things like you. Especially things like the Cloaking Device, where they could "see" you even between the flickers. So other than not adding to damage you're taking, the item was worthless to a human player competing against the CPU.
It's probably a dream (and something players would exploit if they did implement it), but it'd be nice if they had troubles fighting on the extreme corners of the screen (with some levels like the classic Yoshi stage, where the camera doesn't show the corners all that well when the fighters are spread out), when they're forced to fight off-screen, against visual imparement items like the Cloaking Device, and such. Maybe it already does this, but I haven't noticed it if that's the case.
Also, the more (even unlockable is fine) adjustments, the better. I don't like how often carriers and party balls blow up (I've gotten used to it, usually employing the carrier as a weapon more than what's inside it), I'd like to be able to turn that chance ratio down.
Something approaching endings (like the original SSB had) would be nice, too. But not an absolute requirement.