Give Waluigi his own game? He hasn't even made an actual appearance in a big title yet. He was made simply to take up space and serve as a partner for Wario. Plus, Wario has become much more than an "evil clone" in recent years.
I believe Waluigi has potential should he ever be used in a major game, but at the current rate, he's pretty much forgettable, given that all the Mario RPGs thrive on characters specifically created for them, Nintendo EAD doesn't put out big Mario games often, and even then sticks to the old "Save Peach" plot, and Wario has his own series in which Waluigi has yet to appear in.
Also keep in mind the only reason I think Waluigi could be good if used correctly is only based on how he acts the the minor Mario games. The intros to Mario Power Tennis and Mario Golf TT come to mind.