Excluding the ones that are already available as companions, such as Twi'lek.
Having just finished reading "Blood of the Empire", I would like to see... well, whatever Maggot is. One of those green, one-eyed dudes. An expy of Maggot would be an excellent companion for my inquisitor.
Also, one of the adorable tiny blue elephant people (I'm not good with species names) would get along well with my Jedi Knight, I think.
What species would you like to see as a companion, and for which class?