This revelation, plus seeing a bunch of other GameCube stages added, now tells me that HAL's stuffing in a lot of the GameCube content and doing fan service. I wouldn't be surprised if Leon S. Kennedy appears (according to the rumor over at Nintendojo). Every time I hear something new added, it makes me more excited about the game, because now it easily has more stuff than Melee.
And Olimar wasn't much of a stretch after we saw the Pokemon Trainer. We've got some flying characters too... I'll state it again, anyone could appear in this game now.
...however, I've never actually played Pikmin or its sequel. I was too busy playing Melee :P