Wario and Waluigi wouldn't make sense as they're both villains. Sure Wario helped out in Super Mario 64 DS, but only because of his weakness to greed and Bowser ticking him off. Waluigi hasn't done anything major in a main Mario game, but he's the type that wants to annoy the side of good. For example, he'd rather give the Marios a good booting and then laugh like a douchebag at them.
The only time I could see Waluigi working in a main Mario game is as a villain (the mini-boss kind or as a henchman).
The only time I could see Waluigi working in a main Mario game is as a villain (the mini-boss kind or as a henchman).