Mon Calamari
Pros: It's a trap! (you knew that was coming, right?) Already modeled as a companion, fits the 'not more alien than Twi'lek' guideline.
Cons: Not many, although modeling headgear will be 'fun'!
Likelihood: Pretty good.
Probably one of the front-runners, Mon Cals are extremely popular due to the Ackbar association. They were also playable in SWG. Given that Guss Tuno was already modeled as a companion, it's a lot less work for BioWare to do to put them in-game, although they'd still have to cobble a female Mon Cal from the ground up.
Nautolan
Pros: Very popular due to Kit Fisto in the prequels and Clone Wars, very visible as NPCs in-game, particularly early Jedi Knight antagonist Bengel Morr.
Cons: Has not been modeled as a companion in-game yet. Doesn't fit Schubert's 'less alien than Twi'lek' guideline.
Likelihood: One of the three most possible options.
People have been wanting to play Nautolans ever since Kit Fisto Force-pushed some droids on Geonosis and grinned. Despite the whininess of NPC Nik Deleru on Coruscant who sends you off to find his mate (who didn't like being with Mr. Clingy Boyfriend anymore), people have still wanted to see them as playable. Their head-tresses would make it a challenge to properly model them in-game, but it's possible BioWare might do that out of respect for how many people seem to want the species around.
Quarren
Pros: They would be completely new to Star Wars games.
Cons: Not very popular, possibly outside of Schubert's guidelines.
Likelihood: Poor.
The fact I have to explain what a Quarren is for folks who haven't delved into the Expanded Universe is pretty much the nail in this coffin. As much as it might be cool to see the race that shares a home planet with the Mon Calamari, they're never going to be as awesome as Ackbar's people in the mind of Star Wars fans.
Rodian
Pros: They're popular due to Greedo and being playable in SWG. Could fit within Schubert's guidelines.
Cons: I don't recall any of them speaking Basic in-game yet. Correct me if I'm wrong.
Likelihood: Average.
The SWG connection would get Rodians better exposure than anything else, but is it enough to get them in-game?
Sarkhai
Pros: Modeled as a very popular companion.
Cons: No one's heard of them otherwise.
Likelihood: Poor.
In-game canon states that Force-sensitives were so rare on Sarkhai that companion Nadia Grell was the only one of that species other than a rumored child. Therefore, either the game would have to override its own canon or restrict Force-using classes from this species to preserve that canon, and I don't see BioWare doing either one.
Selkath
Pros: First introduced in Knights of the Old Republic. Already seen as frequent NPCs in-game.
Cons: Probably do not fit Schubert's guidelines due to their heads.
Likelihood: Average.
Some folks speculated fiercely after the first reveals of the new Forged Alliance storyline that perhaps the Selkath could become playable. This could have also been reinforced by some screenshots given to us at the Boston Cantina Tour on the flashdrive that showed some of the concept art for more Selkath NPCs. Manaan is mentioned as a place where one of the leading NPCs of the story was located as well. The presence of two sets of Underwater-themed gear might add weight to this notion, but I'm not really sold on the idea that BioWare would introduce this race as their next playable.
Togruta
Pros: Arguably the most popular option. Already modeled as a companion.
Cons: Doesn't fit Schubert's 'less alien than Twi'lek' guideline.
Likelihood: Very.
Shaak Ti from the prequels and Ahsoka Tano from Clone Wars are both massively popular. Add to this the fact that Ashara Zavros is most often the companion I hear Sith Inquisitor players say is their favorite, it seems like a win/win to me. However, lekku are evil in terms of clipping on art assets, so that lowers the chances here. Coupled with Damion Schubert's guidelines, I can see them passing on this golden opportunity.
Voss
Pros: Very popular race, no funky brain-tails to mess with art assets.
Cons: In-game canon pretty much nixes this idea.
Likelihood: Poor.
I like the Voss, no matter how infuriating they can be. They're just weird, and that's cool. However, BioWare is the Story Studio (tm) (yes, I just made that up), so it wouldn't make any story sense whatsoever for a planet of insular people that was just discovered like yesterday wind up with adults who just happened to be off-planet for the past twenty years and went through Republic military training, training at the Jedi Temple, training at the Sith Academy, train with Imperial Intelligence... you get my point. If only the planet's discovery were that long ago, that'd be fine, but alas.
Weequay
Pros: Modeled as a companion.
Cons: Not that popular.
Likelihood: Poor.
The most of what we've seen about Weequays in the game featured them as thugs and enforcers, although there was that one Weequay Jedi Master who asks you to spy on his apprentice on Tython. Still, if you're looking at it from a 'what will make BioWare money' sort of sense, I would be thoroughly startled if this was their choice, because I can't see it doing so.
There you have it, some of the more popular species I've heard in terms of potential new playable races. Which one do you think it might be?