Project M/WMG

Mewtwo will be brought back
If you look at the official website's Character Index, you'll notice that there is a question mark in the "Melee Veterans" section...
 * Possible, but not any time soon. The developers are only willing to pursue additional characters after the finalized version with all of Brawl's characters is released.

Project M Toon Link will be a Composite Character of himself and Young Link, and Project M Pikachu will be a Composite Character of himself and Pichu.
After all, Project M Mario combines the best of himself and Dr. Mario, so this is the logical next step. I can already tell that Pikachu will have Pichu's power and maneuverability without being as easy to launch and suffering Recoil Damage.
 * Confirmed, at least for Toon Link, who is a combination of Smash 64 Link, Melee Young Link, and Brawl Toon Link.