While many have been hoping for Luigi to move from an assist trophy to an actual playable character, it looks like that will not be the case. For whatever reason, Sakurai and the team have decided to keep him as an assist trophy for Super Smash Bros. for Wii U and 3DS.
“Pic of the day. 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.”