In the videos released so far we can see Wario, Waluigi, Peach, Luigi, Yoshi, and a couple flavors of Toad. None of those characters are playable in the original Super Mario 64 (no, not even Luigi ...

The Super Mario Bros ... The dark counterpart to Luigi and the ally of Wario, Waluigi was created for Mario Tennis 64 so that Wario would have a partner for doubles matches.