I can't speak for the rest here, but I've felt for a long time that the game could really use more alien races, aliens that really look alien and not like they could be humans in costumes.
I've tried to work on this myself but haven't been successful in making the time for it.
I'll also point out what Sitters mentioned elsewhere in this forum: If you want to make something new, a quick concept drawing or rough draft might be good to show off first, to avoid putting a ton of work into something that doesn't get approved.
Somewhere there is a list of needed models and graphics, please forgive me as I don't have a link handy at the moment, but it is around here or the wiki somewhere.