To be an echo fighter would mean having the same moveset, like how Daisy, being Peach's echo fighter, has her moveset, but with some different animations to make her stand out more
Waluigi having Wario's moveset wouldn't be fitting because of how Waluigi differs from Wario in both size and personality, he'd probably work better as Luigi's echo fighter if anything, but the best way for Waluigi to be playable in Smash would be as his own character, not anyone else's