I like this idea of upping the ante with galactic political and military tensions. It would be interesting to see the Klingons go to war with the Romulans while not really caring what the Federation does. Depending on the outcome of that war, there could be a shift in the way the alliances eventually work out -- instead of Klingons later being semi-allied with the Federation, maybe the Romulans take that position.
I also think there would be a big, quadrant-wide push for colonialism from pretty much every spacefaring civilization around. You hear about the destruction of one of the founding members of the Federation and what do you think? You think, "Holy crap, if that could happen to them, it could happen to us!" Suddenly the drive to populate the stars is rather more urgent than a scientific curiosity.
Yeah, absolutely. Especially if the Romulans now know their planet is doomed. Its never said whether Nero tipped them off, but if they did find out, it would certainly change their perspective on things very considerably. Their isolation and limited contact policies could be changed.
All sorts of people with various different agendas would be looking at these changed events from different angles and impacted in diferent ways by them. The brief incursion of late 24th century technology into their time has had a huge impact on things, and thats a good ground in looking for new story lines.