Sadly, he's not playable...WAAAAH!
Some part of me hoped that Waluigi would make a triumphant entrance in the new Smash Bros. as a playable character. But alas, it isn't to be, as he has been confirmed as an assist trophy (again) by director Masahiro Sakurai.
Sakurai just lays it on thick, stating in a Miiverse post, "Once again, Waluigi's Assist Trophy joins the collection, which also means he's not a playable character. Just because you try hard doesn't mean you'll make it into the battle."
Wahh, Waluigi lose? Impossible!