Really doubt they're going to have two trainers. It would be too confusing on the character select screen. Much more likely that they'll swap out a couple of the Gen 1 starters. Or maybe the 3DS game would have a trainer of a different gen.
Also doubt that we'll see both Lucario and Mewtwo at this point. They're tonally similar without being exact clones, and I suspect they'd put in a more differentiated solo Pokemon before they'd bloat out the pocket monster roster in such a fashion.
Pokemon aside, who else seems likely at this point? I'd say it's pretty certain we'll see:
-Miis (if they put in Wii Fit Trainer, hard to see how the ever creeping Miis don't make it in)
-Dixie (apparently almost in the last game, and now she's back in the Returns series--but would she be a Diddy clone?)
-Little Mac (based on that so-far accurate NeoGaf rumor, plus the boxing ring arena spotted in promos)
That's three I'd bet on, plus the confirmed Wii Fit Trainer, Villager, and Mega Man. There was a 10 character jump to Brawl (13 if you count the Mewtwo, Pichu, Dr. Mario loss). Can we expect 4-6 more, or should they be taken seriously about the modest roster expectation?
Edit: Also, we can safely assume that R.O.B. is gone, right? Amirite? I would also not be shocked if we saw the end of the Ice Climbers, Snake, and Wolf.