Sakurai's reveals has either been during a Direct to promote a game, or on a release date of the game in a series. Having a release date or Direct related to a series that could get in(for instance, the Pokemon direct that happened not that long ago didn't give us anything at all) is not a guarantee either.
He's unpredictable, but if he does want Mii in, that could be a good time to do it. I do like Mii and find him/her a good choice, as generic characters have been Fighters of Smash for a while anyway. 64 and Melee suggest they're just a species as is, although obviously Yoshi himself is due to be a major protagonist and such, but he is based upon using all the generic colors of the race(which is notable among his various games anyway). So it's some of each design-wise. Pokemon Trainer is completely generic outside of the model they chose, and all of what he says is specifically the same thing any of the Trainers would say. Just about every Pokemon Playable Character is essentially generic, all taking nods from the Anime and/or Movies. Mewtwo may be the sole exception at the time of Melee, since there's one overall.(at least till the Genesect movie, where that is very muddied)