That is Tumblr, sir.
That is Tumblr, sir.
All'a this "replacement" talk gets me irked. New characters have never replaced an older one in any Smash game. If ICs have been dropped from the roster, it's likely 'cause they're incredibly irrelevant to modern gaming and are known for bein' broken with their infinites and other such nonsense. I'd also bet money the decision would've been decided (long) before possible new challengers were discussed.
EDIT: ... S'pose I forgot 'bout Toon replacin' Young Link. derp
I'd say Lucario replaced Mewtwo.
What little data of Mewtwo that exists on Brawl's disc (essentially placeholder stuff in preparation for actual development of the character) implies that Mewtwo was cut from the roster literally instantly, as was the rest of the forbidden seven... though that's a whole other topic. Which is to say, he was cut at the very start of Brawl's development. Possibly before it was decided Lucario would join the cast, most definitely before there was any solid idea of what Lucario's moveset would be like.
That piece'a information sort'a debunks the thesis that Lucario replaced Mewtwo because Sakurai didn't want any clones; i f this was so early in development that no one knew what Lucario's moves would be like, obviously they wouldn't axe Mewtwo to prevent Lucario from bein' a clone, the most common argument for Lucario bein' a direct replacement for Mewtwo.
That aside, we don't know why Mewtwo was removed in the first place. We've never had official comment from Sakurai. Could'a been time restraints, could'a been when it was decided to include PT, or both him and Lucario, which is literally four new Pokemon characters. I'd bet a Pokedollar or two that all'a the new Pokemon characters is why Mewtwo was cut (and is a third reason why Pichu was doomed to disappear forever)... though that's a groundless statement, obviously. As I said, we can't know for sure why Mewtwo didn't obtain that coveted Smash veteran status.
I heard (this may be completely false) that Mewtwo was cut at the very end of development due to time constraints, and they decided to program in Sonic in his place because he had a far easier to design moveset. So, if that's true, Sonic replaced Mewtwo more than anyone else did.
On that note, if Mewtwo makes it into SSB4 (which he very well might,) I wonder if his Mega Evolutions will be incorporated?
If he did, his Mega Evolution(s) would almost definitely be incorporated in some way. They're the only reason why he's relevant to Pokemon at the moment, after all.
He was cut in the very, very beginning. That much is absolutely certainly. There'd be more data of 'em otherwise.