Doc would likely still have Mario Tornado as his down B, for one. I'd imagine he'd still be a bit more powerful than Mario.
Roy would be a combo-oriented version of Ike. Stronger than Marth, but has many of his combo-able attacks (namely a better air game than Ike's).
Pichu would still be a weaker but faster Pikachu, although he might have something other than Skull Bash this time around. Maybe the existing Pikachu would have had Iron Tail?
Mewtwo would be a Luigified clone of Lucario w/o the percentage-based increasing damage, and would likely have specials that stun instead of power attacks and counters like Lucario. His Final Smash could be the blue psychic sphere he had in the first movie, where he doesn't take any damage, and his psychic attacks would become more powerful. A bit like a controllable, slow Volt Tackle (Pika's FS).