I don't know if Sonic is "megaton" worthy. That was more obvious than Sheik's identity. Cool, but hardly shocking.
Can they top it? Easily, I'd say. Pick a beloved, classic character who isn't named Sonic the Hedgehog. A few worthy of mention:
1. Mega Man is an easy choice
2. Belmont. Here's a character no one expects since he's a Konami IP, but most can agree he deserves mention. I think a Belmont would be off the f*cking chain, but then again, I'm a Castlevania whore.
3. Cloud. I won't get into it, but the faces of fans/haters all over the world would instantly melt off.
4. Ryu (Street Fighter). I wouldn't want him before Mega Man, but he'd surprise everyone.
Thing is there are tons of memorable and popular characters to choose from and who wouldn't want their character in this game? And every gamer has a favorite.
If they stick to the supposed 2-3 3rd party characters limit (which I don't believe) and assuming there's a 3rd, I'd bet on it being Mega Man. And my guess is that he'll be shown sooner than later.