Alright, I'm ready to wager a guess. Sheik is back, but is her own character. Zelda and Sheik each have at least a new vB attack. I think Sheik will be unlockable, too. I'm Basing this on the idea that we've now seen the starting Roster, with 21 characters. Sonic and Snake are unlocks, but announced because Konami and Sega wanted to make a big deal about it, and ZSS isn't going to appear on the character select screen, most likely.
That's my guess, at least.