My primary concern is that the gameplay for Smash retains its simplistic yet quasi-technical nature. Smash, at least in its Gamecube iteration, is probably one of the deepest fighters out there b/c it combines standard technical practice (i.e. L-cancelling, teching, DI) w/an incredibly advanced physics and damage system. What results is something that appears to be simplistic in nature, but is actually complex.
Smash Revolution needs to keep this balance of simplicity/complexity. My deepest nightmares involve HAL--or whoever is making Smash--somehow dumbing down Smash for the gaming public. Specifically, I'm terrified that they'll somehow remove wavedashing/L-cancelling and reduce the number of neutral stages for more craziness w/random items and the like. If anything, HAL should follow Sammy's lead w/GGXX and try to incorporate glitches/techniques into their future iterations--as Sammy did w/option selecting in GGXX.
Speaking of GGXX, it'd be great if Smash Revolution had more technical stuff to learn, like different lag-cancelling techniques and mid-air dashing. Catering the game towards a hardcore fighting crowd probably won't happen for all of eternity, but I'd love to have more advanced techniques in there.
I'm worried about online play too, mostly b/c from the looks of things, Nintendo has no idea how to incorporate online properly. Ideally, Nintendo should make an online system for Smash similar to DOA3 (or is it 4. . .?) where you can watch players' matches in a sort of virtual "waiting room" and chat freely w/other players to organize future matches. No more garbage w/friend codes and the inability to communicate w/the people you're playing against. But then again, since Nintendo has never really viewed Smash as a competitive fighter, I'll doubt that they'll make the online system similar to DOA3 in the first place.
So yeah. . .more advanced techniques, keep the balance between simplicity and complexity, and make the online system good. That's all I ask.
That, and give Mario a proper KO move.