A plausible thesis, though I'm still on board the sentiment that Mewtwo was cut early rather than late.
Other returning characters I think are likely to make it are Diddy Kong and Zelda. Jigglypuff could see a new moveset with Fairy-type moves to represent the new type, although other than it's type change it's been pretty irrelevant recently. Lucario could incorporate it's Mega-Evolution somehow, but they might not want to have too many Pokémon characters in the game. I'd also be very surprised it other oldies like Ness and Yoshi didn't see a return, as they are both very unique characters.
Zelda, Ness 'n Yoshi are all veterans, so they'l return guaranteed.
Jiggles won't see much moveset change I don't think. Even once-clone characters (whicharentclonesbutughanothertopic) like Flaco 'n Ganon didn't see much change to their moves in Brawl; I interpret the reasoning as Sakurai not wantin' to put off players of that character by changin' 'em too much. So yeah, I reckon Jiggles will stay almost the exact same, much as we'd all like to see Rest replaced with any Fairy-type move. nick dont even
If Diddy didn't become a veteran by returnin' for Smash 4, I'd be shocked. The Kong series might even be deserving of a third rep (ohai moveset thred) if new games keep on comin' out, not that I specifically predict it happening.
Pikachu never was a clone character though...
It's the other way around...