I'd try to avoid jumping to conclusions, but I'd also have a hard time not thinking of K. Rool as confirmed in that case. More than pretty much any stage so far, the Gangplank Galleon is blatantly a homestage for K. Rool; if they've dug back into the classic trilogy for that specific stage, rather than, say, making the stage a generic Returns pirate ship commandeered by crabs, it'd make absolute sense that they'd bring back K. Rool along with it. It'd be like showing off a Bowser's Castle stage on the Melee DOJO before showing off Bowser himself, pretty much (that stage needs to happen this time as well).
As far as the costumes go, the route that seems pretty agreed-upon at this point is to have K. Rool take weapons from each persona and use them as specials. There's also the intriguing possibility of him being a stance character who swaps costumes and a few attacks with Down Special, which could really fulfill Sakurai's "must do something no one else can" even better than he already does.
Funny, I'm reminded of some of the old anti-K. Rool arguments in talking about his personas; oftentimes, the ignorant will claim "Oh, K. Rool has no personality, he's just a generic evil boss," then, when his character arc of descent into schizophrenia and madness is brought up, it's "Durr, of course he has to change clothes to be interesting." Oy vey.
