i really dont see why anyone would want to play the borg, their function is simple asimilate over and over again. They dont research they assimilate, they dont mine resources they assimilate already built structures into their ships or build new ones with it. They dont talk to anyone, or trade or much of anything else. As a faction, it would be the most boringly repetitive to play.
The dominion on the otherhand could be interesting to play, patroling ones territory and making sure everyone in the dominion tows the line, probably there would be more missions involving spy like infiltration and sabotage missions than not.