So nobody else has to look it up: #24 is Mewtwo. Giygas is an Echo Fighter of Mewtwo.

As much as I would love to have a Mother villain, I have some problems with this:
Giegue powers: In the Mother series, he represents an unstoppable force, invincible, impossible to understand, all the way back to the first Mother you couldn't defeat him unless you sang a song, and you didn't really defeat him, he just run away crying, Trying to make him a normal fighter is kinda against his character and what he represents.
Mewtwo's echo: There has not been a single Echo of a character that was from a different franchise and now that we know echoes can share the slot of the original ones makes everything even harder, having a pokemon rep sharing slot with a mother rep, having a mother rep between all the pokemons, I don't know… I don't see it happening.
With all this being said, I don't oppose to the idea, but I would honestly like it more if Porky was the mother villain and then he had the ability to summon Giygas or even use him as a final boss, that way you would have both Mother main villains in one character and a common rival for both Ness and Lucas.