Tbh, I don't we need any teases from Sakurai regarding Xenoblade content, and Shulk being playable. The announcement of a Xenoblade successor is all you need to make you think, "Shulk's in Smash". As soon as the Xenoblade logo appeared, Shulk's chances increased dramatically. The nods and ties to Xenoblade in Xenoblade X speak for themselves.
In the small chance that Shulk isn't playable, it would be really, really strange to not rep Monolith Soft since they are one of Nintendo's hottest devs, and the only Nintendo subsidiary who doesn't have a rep. Monolith Soft is not making small eShop games here, they're being given the go-ahead to create massive scale, open world HD RPGs; Nintendo obviously believe in the talent at the company and trust they can pull these kinds of games off.
The Xenoblade Streetpass puzzle (sounds dumb, I know, but it's more important than you think), VA tweets, Monolithsoft/Nintendo's relationship, too. Man, I'd be legit surprised if Shulk isn't playable. If Next-Level Games can get a rep via Little Mac, it's impossible Monolith Soft can't get a rep.
Pretty much all the newcomers revealed so far are from the Wii/DS era. Going by this, there's no way in hell Sakurai is going to overlook a critically-acclaimed, relevant, Wii era RPG from which its respective protag is one of the top requested and has a successor in the works, and is developed by a Nintendo owned development studio. You'd have to be nuts, and a complete prat, or both.