"Star Trek Online" offers an interesting premise for another series; Romulus and Remus destroyed, with the Romulans forming rival factions. Starfleet has ordered that ships help in any way possible with civilian Romulans.
The Klingons, seeing an opportunity for invasion, enter Romulan space and claim several systems as there own. Conflict is inescapable, and the Klingons break the century-long treaty with the Federation and attack Starfleet.
The Borg are reading there forces, to return in the 3rd or 4th season.
The Dominion, the Tholians, and the Cardassians would play in eventually as well.....