Romulans would be more of a threat than ever before. Do consider that they are a aggressive arrogant race to begin with: when word has reached their planet that their sun is going to go super nova, the campaign for expansion has switched from ego tripping careers to a necessity for survival of the Romulan Empire.
I'm still kind of dumbfounded as to why STID completely shelved the Romulans even though all the problems in this movie was a direct response to a catastrophic Romulan attack. I know Nero said he didn't speak for the Empire, but the Klingons have never destroyed an entire planet before, and they were also attacked by the Romulans in the same movie. So to shift the focus to the Klingons and ONLY the Klingons seems really out of place.
On one note: The Federation is made up of different planetary alliances, so perhaps other alien cool heads has prevailed? Most likely in settling any affront to an allied planet, one should not respond in anger would be the ongoing practise in keeping the peace among themselves and not just how they will respond to an outside attack.
On the other note: I am under the impression that Klingons were a closer threat to Earth than Romulans were. Perhaps it is by proximity is how a threat is assessed as being greater?
The further the distance, even if Earth was conquered by Romulus, the greater the chances that their resources would not be able to maintain a conquered planet, and would be at risk by the native insurgents on the planet's surface as well as their supply line being open to Klingon attacks.
It would explain why Romulans have been so busy attacking the Klingon Empire so much.
Still, you would think we would hear of these skirmishes in the second movie to paint the atmosphere that the Romulans were going full tilt on this now that their solar sun will be going supernova, thus giving a hint of the 3rd movie.
But in order for the Romulans to be a threat, the Klingon Empire has to be crippled enough so Romulans can proceed to attacking the Federation, with the covetted prize being Earth.