@HHShetland
I'm thinking of leaving non-human characters on the sidelines for this one (I have a feeling aliens will be exceptionally popular and the crew will be swamped with all sorts of creatures and it will turn into a alien melting pot as opposed to a platform for a series of diverse characters (as opposed to species) to interact).
You sure that's a good conclusion to reach? I mean, it all depends on the writer. Human characters are just as vulnerable to the curse of unimaginative and boring character as non-humans; if not more so, since the writer doesn't have to create all the fluff surrounding the creation of new species...es. Likewise, it's perfectly possible to make well-written non-human characters. I like to think I always make an attempt to give my non-human characters... well, actual character (as opposed to a Planet-of-Hats style stereotype).
In addition, I feel as though your predictions are inaccurate based on my experience with RPs. From my experience, even if you allow non-Human characters, most people will still pick humans because they know how to write them better since they themselves are (presumably) human, and that's perfectly fine. And a lot of the people who aren't human are creatures that look so much like humans that they might as well be. As a result, only a small handful of truly alien characters come into the picture, which I'll admit may make it easier for an unimaginative writer to just throw character to the sidelines and make them into a token alien stereotype, but at least they wouldn't detract from the overall quality too much, assuming that all the human characters are interesting enough. And if the alien character(s) are interesting too, then that just makes things even better, if you ask me.
Anyway, I just thought I'd give my opinion here. I understand your concerns, though; personally I'd recommend limiting the amount of non-humans rather than banning them altogether, just to avoid alienating, no pun intended, certain types of RPers. Though if you're still not convinced, that's fine. I never play as humans, so this might be a good opportunity to experiment.