Eh, I never really considered Meta Knight unlockable content, but I'll agree to disagree.
Saki is a great choice for an E3 2012 reveal. Big names don't mean everything, SSBF. Strong marketing is where it's at. Saki isn't well-known, but he'll be well-known if he gets into Smash. And casuals aren't going to care if they don't know him, I can guarantee you that. They'll just want to play as him because he looks cool or not play as him because he doesn't. Plus, he's perfect marketing material. A teen with laser guns and swords fighting the likes of Mario, Samus, DK and Pikachu? What marketing company in their right mind wouldn't want to snatch up that opportunity?
Smash is at the point where it already has a large, dedicated fanbase, so it doesn't need big names to sell itself anymore...