This is a pretty wild guess but looking in the Star Wars: The Old Republic Encyclopedia, the way the species section is presented, after Cathar the next few playable species could be, in no particular order: Togruta, Chagrian, Devaronian, Advozse, Mon Calamari and Kaleesh.
What kind of annoys me with Cathar is I remember reading that it would be a Legacy species. If this means what I think it means, then Cathar would only be accessible after reaching a certain level. To me, this would be pretty frustrating especially to the people who wants to play as their favourite species from the start only having to play as one they don't want just to obtain another. I would understand if like the rest of the current selection it was available from the start but only to a couple of classes and then unlockable to the rest via the Legacy system. I guess we'll have to wait until either 1.7 or 1.8 gets closer to know the exact details on how they'll handle them.
Well, now with the free to play stuff in place, you could probably get them from start with cartel coins.