A couple of weeks ago, there was chat in one of the official Twitch streams hosted by Star Wars: the Old Republic's community team where members in the chat were stating that a new playable race is confirmed for the game. We've known that the Cathar introduced in Game Update 2.1 weren't ever intended to be a one-off addition to the game, but their success or failure would prioritize future species options. It's been over a year since the Cathar came in along with the controversy over the fact they were a 600 cartel coin purchase, and there hasn't been a lot of rumbling in the ether regarding a new race until fairly recently. Back at the Cantina Tour stop in New York last October, Gaddock Teeg reported that BioWare's Lead Designer Damion Schubert said that a new race in the works was 'a little more alien than Cathar, but less alien than Twi'lek'. Let's assume for now that this guideline hasn't changed in the months since.
We've also known for the longest that player characters would naturally have the most modeling done to them for the purposes of checking out how gear would look on them. Companions were second, simply because they also had to be checked out against the gear library, other than exceptions like Khem Val. What that meant was that any companion race was more likely to become fully playable, simply because it would be less work to bring them in versus taking a non-companion race (say, Nautolan) and putting them through the 'how much clipping are we going to see here?' grinder. There's also some hard ground rules about what races could or couldn't be added to the game. Due to the voice acting present, there could never be a new race that could not speak Basic. Sorry, Wookiee fans, but despite the fact that Wookiees were playable in Star Wars: Galaxies and the Smuggler gets one as a companion, they'd have to rewire all of the voice acting to accommodate them in SWTOR. Same with folks wanting to play Trandoshans in homage to Qyzen Fess, or those of us crazy souls who think it would be pretty awesome (if a liiiiiiiittle bit lore-breaking) to run around as a Jawa Jedi Knight. UTINNI! *womwom* Unfortunately, another strike against shorter species such as the Jawas or Ewoks is the simple fact that the game's cutscenes would need to be redone to fix the eye-line.
However, there's still plenty of humanoid races to look toward in terms of a new playable species. Let's go over some of the possibilities.
Pros: Popularized by the Dark Horse comics with Jedi Master Arca Jeth and Jarael, easy to model.
Cons: Not really too alien, but might qualify per Schubert's comment.
Likelihood: Fairly good possibility.
BioWare has never had a problem dropping Old Republic comic lore and gear into the game (both Fleets are officially named after characters from them, after all). Interestingly enough, the Arkanians could easily bring folks who like 'pretty' characters back to the table. Whether you're dealing with a white-eyed/non-pointy-eared/caucasian flesh-toned pureblood or the pointy-eared/blue-eyed/tattooed/white-skinned/white-haired offshoot, the folks who code up the character generator could accommodate all of those options. You can almost make a pureblood Arkanian by picking Human in chargen and get the white eye color unlock, but there's not currently a way to make an offshoot like Jarael.
Pros: Popular in the EU, already modeled to some extent in recent game content as NPCs.
Cons: They're a bit more alien than Schubert's comment seems to limit.
Likelihood: Not very likely.
Many Bothans might have died in the future, but for now, their popularity doesn't seem to be meteoric. They're also pretty heavily Republic-favored in most of the primary canon, so many players will have a harder time considering them as legitimate for Empire side, same as some players won't play a Sith Pureblood on the Republic side due to breaking lore.
Pros: Already modeled with Jedi Knight companion Sergeant Rusk.
Cons: Not hugely popular lore-wise, the most famous canon example was Palpatine crony Mas Amedda.
Likelihood: Not all that likely.
Pros: Already modeled with Bounty Hunter companion Gault Rennow, fits Schubert's criteria.
Cons: Not hugely popular.
Likelihood: Fair to middling.
Pros: It would be new. The HK droids are stupidly popular. Has been modeled as a companion (SCORPIO).
Cons: Droids can't wield the Force, so they can only play the four non-Force classes.
Likelihood: Very poor.
Here's the thing about droids. Yes, they'd be massively popular if you could play one. The Cybernetics series of armors seemed to sell well on the cartel market, so the modeling has been done, and there were roleplayers out there putting on that gear as Cyborg species characters and actually RPing that they were a full-on droid. However, they could absolutely not be Force-using characters. The concept doesn't exist in any vaguely canon Star Wars lore. The closest you get to 'robotic being that wields lightsabers' is General Grievous, and he's a Kaleesh cyborg, not an actual droid. And he too isn't Force-sensitive. BioWare couldn't release a new race that half the game's classes couldn't actually be, it wouldn't make any business sense for them. So as much as it might be a cool concept on the face of it, I wouldn't bet any money on that.
On the other hand, if BioWare really wanted to do something innovative, they could go back and rewire the Cyborg class to look more like how players look wearing the Cybernetics gearsets, rather than 'human with metal on their face'. This would also be appealing to free-to-play players, who can't play anything but Human, Cyborg, and Zabrak without unlocks or upgrading their account.
Pros: Fits the 'alien but not too alien' limitation.
Cons: Not too popular. Was never modeled as a companion.
Likelihood: Middling to poor.
This would likely be an example of someone phoning it in. The Duros are there all over the game as various NPCs, but there's no real spark to justify the cost and expense of turning them into a playable race.
Pros: Um, modeled as a companion.
Cons: Oh god the property damage.
Likelihood: Nuke this idea from orbit. It's the only way to be sure.
Pros: Very popular.
Cons: They don't speak Basic, never modeled as a companion, doesn't fit within the 'not too alien' guideline.
While the Ithorians are madly popular sometimes and were playable in SWG, I don't really see them becoming playable in SWTOR. After Master Quiljayk on Tython got a facelift and looked more like a Jedi and less like a random spacer, the 'must speak Basic' rule will unfortunately rule them out more than anything else. Pity, because I really liked that Ithorian Jedi's robes quite a lot.
Pros: Already modeled via Xalek, popularized because of General Grievous.
Cons: They only exist in-game because of Grievous.
Another example of possibly phoning it in, the Kaleesh make a logical option because of Xalek, but I'm not really sure it would be a business-wise decision for them. There are more popular options with a similar amount of prep work involved.