Races could be , Cardassian , Romulan and Reman (of course). Then Hirogen and Alien and maybe Alpha jems in c-store.
Cardassia is on the opposite side of the sector, heck they are not even in the same Quadrant and in order for Romulans to reach Cardassian space they would have to go across Federation space, heck considering you are locked out of Sirius in the game you would also have to enter Klingon space.
There is no reason to have Cardassians on the Romulan side, they dont share borders and are too far away to even be able to provide assistence, it would make more sense to have Cardassians becoming part of the Klingon Empire as that did happened in a way in the Mirror Universe but it would be very ... awkward because there are significant Federation territory in the way and will remain to be so until Cryptic adds a new sector.
Also give me a break with the Alpha Jem Hadar, they are loyal to the Changelings and will always be so since they were altered to be so along with their ability to reproduce naturally, they are either on the Dominion or in the New Link.