Well in the Original Series the Romulans were a race of 'honorable warriors' much like the Klingons later became. The Klingons in the Original series were much more like Cardassians, an almost Orwellian society, backstabbing, treacherous and unpredictable. I think if the later series had followed the original model we would have seen a 'Next Generation' featuring the Romulans and Federation as allies and the Klingons remaining the adversary.
It always seemed to me that the Romulans had more in common with the Federation than the Klingons ever did. In the Original Series it seemed like there could have been peace between the Federation and The Romulans if either side had been able to put aside thier mutual distrust and resentment from the previous conflicts.
The Klingons on the other hand were brutal sadists who used thier technology for hideous purposes like the mind shifter, and agonizers who believed that sick people should die, and weak people had a right to be conquered.
As for the Romulan Empire itself, I never saw it as some vast far reaching swath of territory, at least not as of the 23rd century. I thought perhaps it would consist of only a single solar system or two and that any other worlds they had annexed during the war had been liberated and were then protected by the neutral zone surrounding Romulan space. Romulan innovation has allowed them to use what little resources their system has to keep their military a force to be reckoned with, but there is an ever increasing need to expand their territory.
This likely culminated in some sort of conflict in the era of the Enterprise-C which lead to the "Tomed Incident" whatever that was all about and a need to renegotiate with the Romulans in the interests of peace.