The problem is that you are dealing with an enemy.
TOS - Romulans are the enemy
TNG - Romulans are the enemy
Voy - Lost In Space point is moot
DS9 - Even when the Romulans were on the same side as the Feds they still were not to be trusted.
I think if you want the Romulans on a Hero ship you are going to have to do it in FanFic.
The Cardassians were the enemies in DS9, you still had two major reoccurring Cardassian characters. Heck the Domionion was also the enemy, but they still had Weyoun as a major reoccurring character.
In Voyager, the Borg were the enemy, but 7 of 9 was still a main character.
There are ways they could have had a major Romulan character even if the Federation was at odds with them. What about a Romulan commander who is a Romulan counter part for Kirk or Picard, tasked with dealing with them and becoming their nemesis over time. Tomalak was supposed to be that in TNG for a while, but he didn't get enough screen time. He should have appeared more, instead of the random Romulan commanders they often had. Or they could have had a Romulan exile helping the Federation. I think they could have had a major Romulan character if they wanted to, but they never seemed interested.