Normally, I'd have said the Cardassians as a 4th playable species, but honestly, I don't think their current empire is in the correct state to be of any substantial threat to anyone just now.
Why would that be important? Everybody cooperates at endgame anyway... for fighting the Borg, the Undine, the Iconians, Rebel Klingons, Tal Shiar, or any of the other endgame opponents. So why not have Cardassians in Task Force Omega and elsewhere.
Edit: I wish people would quit asking about a Borg Faction. It wouldn't work. [...]
Agreed. Apart from story reasons, seriously: A Borg player ship in an STF?