All this pro/anti talk about Waluigi and I'm not sure what to think. On the one hand, yeah, he could be a really fun edition. After Paper Mario and Fawful, he's my most wanted Mario rep.
... Yet there'd be something innately wrong about Waluigi getting into Smash. It's fundamentally to do with his character. Look how Wario is a corrupted and grotesque version of Mario. For Mario, it's all about saving the princess and possibly saving the world. On the way he may collect a load of coins, but that's hardly his end goal. Yet for Wario, getting money and treasure is what it's all about. If he happens to save a princess or a kingdom then whatever, but it's more of a side effect of his actions. This is most blatant in a scene where he literally chucks the princess he just saved to the side so he can claim his treasure.
What I'm getting at is that a similar situation goes for Luigi and Waluigi. One of Luigi's personality traits is his resentment at being overlooked and constantly living in his brother's shadow. Waluigi takes this to an extreme: he's self-pitying, thinks everyone else is cheating and yearns more than anything to be in the spotlight. Recognition to Waluigi is what money is to Wario. By including Waluigi in Smash - which is the Ultimate (har har) form of recognition - that would kind of go against his character. Yeah, it'd be nice for Waluigi to finally get what he wants, but then what could he be self-pitying and resentful over? The character is just a punchline to Nintendo and, as sad it is to see him constantly get dunked on, that's part of who he is.
... I dunno, probably overthinking this