Forget the tennis racquet, give Waluigi a completely original moveset that's more based on his personality rather than stuff he's done in the games.
Things like tricks and gadgets, spring-loaded boxing gloves, fake item boxes, a lot of deception and chicanery. Have him bend the rules of the game. I imagine his whole gimmick is that he's a cheater so he uses the limitations of the game against the people playing it. Have him reach into the game's code and pull out a random item for himself to use. Have him hit them with an attack that, in certain conditions, will temporarily disable a button on the opponent's controller, or reverse the opponent's controls. Have him spit poison in people's faces.
I'm just saying, I'm not a huge Waluigi supporter or anything, but I think you guys have a character with a bit of interesting potential and you're going about him all wrong.