Star Trek is full of people and factions that are not part of the federation or any massive empire. Yet, I see no plan so far to let us play as, say, Yridian smugglers or Maquis rebels.
While I agree we should have the Cardassian Union and the Romulan Empire as playable factions, and probably even more, I think this leaves a lot of species out of the game. And it would be a shame if we wouldn't be able to play them.
What I propose is, instead of making seperate factions for all these different species, we could have a "non-faction" faction with neutral bases like Drosana. With weekly episodes and UGC on the horizon, leveling shouldn't be much of a problem either.
I won't even go into detail about the actual species in this faction, because there is a huge amount of possibilites. But the species we currently have on the Federation and KDF side should be playable here as well.
I realize this would not be on the to-do list anytime soon, but I think it's worth considering before adding new factions to the game.