This probably is completely unrealistic, but I would LOVE it if Mega Man's Final Smash involved calling the stars of the other series. Like, Mega Man uses Time Stopper to freeze everyone, and X, Trigger, Zero, Model X, Hub, Geo, and Over-1 all come in and blow the crap out of everything in Mega Man's line of sight in either direction (X totally needs to have the X1 buster upgrade for this, BTW). Would probably slow the crap out of the 3DS version, but it'd be totally worth it.
As for music, MM2 nostalgia isn't going to get much better than the trailer, so as long as we're still dry-humping the NES, let's see some MM3 lovin'! Dr. Wily Stage 3/4 and/or 5/6 would rock all, as well as the Fortress Boss theme. All of that would go well with the whole Wily Castle/Yellow Devil shpiels already shown. I also damn well expect MM3's Get Weapon theme.
If we see some inter-series music, I'd like the X2 Intro Stage, and Hell's Gate Open for Zero rep. Legends...who cares what I say, everyone insists Flutter Vs. Gesselschaft is the only one.
And of course, Rockin' On from ZX.
Assist Trophy? Nothing. Seriously, you grab it, and nothing happens.
Figure the rest out.