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.

I think the “it's something not discussed with non-Klingons” Klingons from TOS will return and we will learn that it wasn’t just a 4th wall throwaway joke about makeup budget, but actually a species schism that resulted in an exodus of half their race. And now they’re back, and of course they’re pissed.

Q uplifts the Pakleds.

I think it will be the Syndicate. The Federation probably wouldn't move in to take over world's that the Romulans can no longer handle. I'm sure some of those would fall to the Klingons, but I think most would just kind of be on their own in chaos, which would be a great opportunity for the Orions. Plus, Lower Decks is going to be expanding on the Orions next season

My guess is the Tribbles expand to fill the void. Unbeknownst to most, the Romulans were keeping them at bay. The Klingons had considered them a Mortal Enemy of the Klingon Empire, but was so embarrassed by that, that they decided to give up. The Romulans were all that would stop them.

consider posting this to lemmy.world/d/startrek as well, since that is now up and running

The Klingons, yearning for the Glory of the past, join forces with the Gorn to ignite a war fiercer than anything the galaxy has seen. Or at least, I think that would be cool.

I'd be kind of looking at a Cardassian-led counter-federation. Maybe the Gorn, maybe some others.

What would be interesting is if it weren't an overtly evil "rah, rah, we gonna beat up the UFP and reclaim our glory!" so much as a "with the Klingons firmly Federation allies, the Dominion crushed, and the Romulans devastated, the Federation is unquestionably on top... and that kind of scares us. We're going to band together because otherwise, the Federation could do whatever it wanted with us."

WHERE ARE MY MEMES?

strange, lost/confused Star Trek Online-flavoured squeaking noises

Ok, since no one else has mentioned it, I’m going to mention how the Typhon Pact formed in the Relaunch novelverse.

Different cataclysm, but divided remnants of the Romulans joined up with the Tholians, Breens and assorted others to establish a competitor to the Federation, which was seen as failing its perimeter members planets and species.

Emerald Chain

Maybe i'm missing context, but in which series does this apply?

The Borg as we know them in that time period are gone.