Without it's capital world, the former Empire will splinter. Occupied worlds with non-Romulan indigenous people will attempt independence, some seeking foreign assistance in the effort. Surrounding powers crave pieces out of the outer portions of Romulan space. Various Romulan planets with large populations each declare themselves the new Romulan capital, the fleet commanders choose sides.
After a few years there will either be multiple small Romulan Empires, or a civil war for power over what's left.