New Smash Brothers!

[Sol]

Willing to betchya that there will most likely be platform exclusive characters if they are going to be released on both consoles.

I'd really dislike that. Unlockables that you can only get through connecting each game, however, would be really neat. (Like only getting the Pictochat stage on the WiiU after connecting to a 3DS, get it???? There are more creative ways to do it though, obviously.)

As for new characters, I'd really, really like an Advance Wars (or just Wars I guess) representative. Specifically Andy, considering he's arguably the mascot of Advance Wars.

Aaaaaand as for third party characters, I suppose Marco Rossi, from the Metal Slug series? Pretty unlikely addition, but he'd definitely work, and I can dream.