I wouldn't expect it at all. I'm not a lore hound but I was under the impression that becoming a mind flayer totally erases your previous personality and memories from your previous life. It'd be like an entirely different game, and even if it happened at the end would represent such a strong tonal shift that I'm not sure that some players would like it. I know I wouldn't. That said, maybe as part of an evil playthrough it could be interesting, but I wouldn't count on it being included. Illithids are supposed to be otherworldly and alien and should stay that way.
