Hmmm, I can't see Klingons being reduced to background players, it stands to reason they'd move in on Romulan territory. Infact many races would probably move in such as the Breen, perhaps the Tholians. The whole of Romulan space would become a free for all with the remnants of the Romulan Empire trying to hold them off. The destruction of Romulus is more or less a destruction of the Romulan Empire itself, which could potentially mean all treaties are null and void, so the Federation would be free to implement cloaking technology on all their new ships which could make a good plot device.
It could be that there is an enemy on the otherside of Romulan space that we don't know about that was kept at bay by the Romulans. Now Romulus has fallen this unknown race could be one of the empires that decide to move in.
My own imagination has the Klingons responsible for the destruction of Romulus. Generations and ST3 showed them interested in system-destroying weapons. ENT established that there were different classes than warrior-classes. Imagine where the scientists and the warriors ally to make massive technological progress for military ends (a la America during world wars). If you start with a supernova inducing weapon, it's not that hard to imagine taking it to the next level with a sector-killing supernova. I'd love to see a story with that as a backdrop (Klingons turning genocidal after too many incursions from the Romulans, the Federation conflicted over its values telling it to betray a friend to help an enemy and later how to handle an empire that is more advanced scientifically/militarily.