I could support the idea of Gardevoir as well, but if it means to be the successor of Mewtwo then disregard that. Mewtwo has far more reputation/experience in the franchise... Gardevoir's more of a "cult hit" kind of Pokemon.
Plus, unless you cannot think outside the box, Mewtwo could also pull off an amazing moveset. Go figure for people to allow Melee to set his moveset standards over thinking he has a chance of gaining speed, power, and overall a fresh makeover.
There's more than enough space for more psychic Pkmn. It's not like they'll all be clones of one another spamming Psychic across the field or anything.