I highly doubt Final Fantasy Characters will be playable, but you never know! They might come up as an assist trophy. (Heh.. Go Cloud! Omnislash!)
Anyway, Sonic is in because of high demand, Snake was begged in by the creators of Snake for Melee, but because it was about to be released, they waited for Brawl to put Snake in. I hope Megaman isn't in, I wasn't a fan of his much.. but if he is in, then he'll probably play like Samus.
Also, I think Shadow should be an alternate costume, it fits the theme more, because there's a shadow link as an alternate costume, a white yeti version of Donkey Kong, and a Flame Suit Mario costume, so why not? And perhaps the white hedgehog while were at it.
I hope they take out Pichu/Mewtwo, because they have the Pokemon Trainer, and then they could put in like Scyther (scizor as alternate costume) as another character. I don't like clones too much, but.. I love Luigi xD
OH! And Shiek better be in there.. no one knows yet, because they haven't released Princess Zelda's (B) Down move. So I hope that she's in.. =P
-Krobe