Reposting what I posted in Ten Forward since it was an attempt at a serious answer anyway:
Since only the Romulus system was destroyed, presumably there were plenty of other planets in the empire inhabited with Romulans and with plenty of ships to defend themselves with. So the next big player could be… the Romulans.
I’ve been thinking about this- despite Ni’var being re-unified, there might still be a Romulan empire of some sort made up of the remaining planets the Romulans controlled.
Also, I assume the Romulans didn’t bother evacuating the Remans, so they’re probably mostly extinct.