Well, he has a mouth, so it would be nice for SOME kind of sound to come out of him. For some reason, I keep picturing him with a robotic Toon Link -sort of voice, though maybe something a bit more deep would suit him better. And no actual words, just some "ooh" and "aah" or whatever is necessary on a battlefield. I do like the wooden sound idea, but I guess that would occur when he's hit or something.
...but yeah, I can't get over how great of a character he could be, not only in Smash but otherwise as well. If only all of his rights would/could be acquired, he has so much potential, even if he would just appear in Mario games. Imagine Geno in Mario Kart for example, or a cameo in a Mario platformer like Rosalina. Rosalina and Geno work well together too with the mysterious space/star vibe they have, and popularity wise I'm sure that Geno would reach Rosalina's level if given the opportunity.