Well... Luigi makes sense, but that's already getting old... Wario is overrated, but still underused... Yoshi was already there, classic foes... well, I don't know why they weren't, but what can you do... As for the "new playable character", I just got this sharp flashback all of a sudden about hearing something about there being a new playable character in SMS or SMG a long time ago...
Eh, let's go with Wario. And on a separate note, there won't be a sequel, but controlling FLUDD with the Wiimote... it worked okay in WarioWare, but the real thing?