At least with Player Character the options for his/her attacks would he easy. Use the fishing pole for ^B, shovel for vB, I guess bug net for smash B, dunno for nuetral, and their [whatever replaces the Final Smash] could have the train or taxi storm through.
On the other hand, Tom could pay the other characters off, or hit them with his massive tanuki scrotum. And that would be great.
I don't really know of anyone new to put in, though I like the idea of an Advance Wars character, but I hope they make it so you can save the setting for match types, and be able to name them. Then you can just hit one thing to play by either stick-in-the-mud type rules (Fox only, no items, Final Destination), or kooky, no-idea-what's-happening.
Also, I hope they keep R.O.B., but that's mostly because that's who I'm best with.