Mewtwo will be brought backIf you look at the official website's Character Index, you'll notice that there is a question mark in the "Melee Veterans" section...
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.
Kirby would be closer to his N64 incarnation.As Melee!Kirby isn't competitively viable.
Kirby is being withheld until all other characters are released.For the purposes of his copy ability. He has more attacks than any other character in the game, and with the Clone Engine perfected thus enabling even more characters to be added, he will not be easy to program at all.
A custom announcer will replace the existing one in v3.0.This will be kind of a necessity with the Clone Engine adding characters that don't exist in Brawl.
The Fighting Alloys will be removed in order to add four more possible character slots.Multi-Man will be reprogrammed to generate recreations using the Alt. Costume engine.
The Subspace Emissary will be disabled to add more stages.It's already being pulled apart - look at Duon and Galleom, who shout at you in Japanese and laugh at you while hopping madly the whole battle because Roy and Mewtwo's sounds had to replace something. It stands to reason that if more characters are added, the other bosses will be the next things to go. So it stands to reason that other things may be taken out as well. It's possible that a large number of stages can be taken out and added, perhaps to get the full array of stages.
The remaining five character slots will be taken up by:
Ridley may be added with the Clone Engine, using Charizard as a base.
Waluigi will be an character
In the next major update, the current clone engine will be abandoned in favor of its successor, the BrawlEx Clone Engine.