Hmm. You're definitely right. An alien companion would be awesome. But then, I guess kind of the point of companions(to the writers, not the Doctor), is to let us connect with somebody on the show, instead of just having to deal with the doctor's weirdness all by our lonesome. Aliens could still do "Holy crap what is going on" exposition, but maybe we couldn't connect to them as well...you know, that theory just doesn't hold up. I would love an alien companion. Everyone would love an alien companion. I might actually start watching more of the show. But yeah, definitely more human diversity would be nice at least.