Professions: The professions would likely revolve around the three main specialties of biotics, tech and combat. Biotics is an important part of the Mass Effect universe, so it stands to reason that a player could become and expert with Element Zero and the bio-amps used to augment the abilities. Biotic implants would be a key element of research, limited of course by the timeframe of the story. The Asari could get bonuses to the profession.
Tech could get specializations in the creation of omni-tools and synthetics, which could then make for some interesting additions and upgrades to player gear. As in the main Mass Effect games, decryption, hacking and electronics could be starting benefits or unlockable abilities with more skill levels. Players could also create various other tools or gear that would supplement the basic tech abilities.
Combat expertise would necessitate a study of weapons and armor. Imagine a buffed version of the standard N7 armor, or weapons with better optics and damage.
These professions would be closely tied to the class the player chooses, so a biotic would not have access to combat professions except maybe at higher levels when a hybrid class can be created like the Vanguard. There could be secondary professions added for flavor, learnable by all.
Talents and Skills: Again, this is an area that could be pulled from the three games with some tweaks and balancing. The Mass Effect series has a rather robust list of abilities for each class, allowing for a multitude of play styles. It is likely there would be few changes or additions, except for the need to balance these abilities in a PvP environment to keep any one class or hybrid from being overpowered.
PvP possibility: Mass Effect is a conflict-based game, so it seems likely that some sort of PvP would be implemented for the MMO. Mass Effect 3 already had cooperative multiplayer, but did not allow for taking on human opponents. BioWare has gotten its hands dirty with PvP in Star Wars: The Old Republic, including the recent implementation of arenas. The functionality could be transferred to a Mass Effect game, but several passes would have to be made at the talents and skills (as mentioned above) to keep any one class from being overpowered. As any experienced MMO player knows, PvE specs and PvP specs are almost always radically different. And since PvP would be a new addition to the ME universe, a lot of balancing would need to go into it to make it viable and fun.
Subscription vs. Free-to-play: BioWare has moved away from subscription in SWTOR, so a free-to-play Mass Effect MMO that uses its Star Wars kin as a guide seems most likely. Microtransactions, upgrade and gear packs, and the ability to purchase an in-game currency such as SWTOR's Cartel Coins already has a foundation. So you could expect a F2P game with total access to all aspects of the game, but limiting weekly instances, bag space, number of account characters, or certain abilities and benefits. BioWare is still tweaking its F2P model, so depending on when a Mass Effect Online game would be released, the model could vary a bit based on player response in beta.
Likelihood and Prediction: In early 2012, before they left the company, BioWare docs Greg Zeschuk and Ray Muzyka had briefly discussed the possibility of an MMO in the ME universe. "The implication of a Mass Effect MMO has so many expectations," Zeschuk said. "We already bit off a big thing to chew with a Star Wars MMO, and that's not so small. It's daunting, but the neat thing is it would lend itself to a different type of game play. It's fun to think about. I imagine people think it would be just like Mass Effect as it is ... but there's lots of people there. It's really interesting."
Muzyka said that the expectations would be huge. "When you deliver a game, and you deliver it for a player, you have to capture what they think is the possibility space," he said. "You need to let them do everything they think they should do, and you can't block them from doing anything they think they should be able to do. You have to nail all the features and content that should be in that possibility space. Mass Effect is a big possibility space."
So it is obvious BioWare has considered the possibility. The company created the material, so it has an even better foundation for making a more compelling MMO than a licensed IP such as SWTOR. I also expect BioWare to try even harder to get it right by fans, no matter how fickle and opinionated they may be (see the controversy surrounding Mass Effect 3). That said, much of the groundwork is already laid. However, the company is enmeshed in the new ME game, a Dragon Age sequel, and continuing to build out and run SWTOR. I think we will get a Mass Effect MMO, but not before one of those projects starts to wind down, or Electronic Arts brings on more people for a whole new team.