[Nate the Great]

Let's look at all of the Klingon episodes and see which ones would've worked with the Romulans as villains:


Errand of Mercy: Not a chance. If the object is to use Organia as a beachhead for the war, the Romulans would just eradicate the local population. In general I don't see the Romulans as the kind to take over the government. They'd just impose tributes, maybe even just set up a garrison where there aren't any Organians and avoid the hassle.



Friday's Child: Probably not. They're too sneaky for something this overt.



The Trouble with Tribbles: No way. Romulan mentalities wouldn't fit the plot, and I don't see them doing something as crude as a bar fight. Instead they'd attempt to fan the flames of the Kirk/Barris feud.


A Private Little War: I guess. However, if I were the Romulans in this situation I'd pose as Vulcans to sabotage Federation efforts. Not just introduce guns under the guise of natural development, but sow distrust of our heroes by posing as Enterprise crewman.


Elaan of Troyius: The script would need a major rewrite, but I think it's possible.



Day of the Dove: I guess. The big difference would be that I think the Romulans would eventually see the logic in stopping the fight rather than letting blind pride ("We need no urging to hate humans!") make the decision.


The Savage Curtain: No way. Colonel Green already had the sneaky, methodical villain slot covered. A Romulan would be redundant, although I do wonder what the Romulan equivalent of Khan, Green, etc. would be like.


As an afterward, the Romulans could've easily replaced the Orions in "Journey to Babel." Maybe have the assassin pose as Sarek's assistant. It's not like Vulcan makeup is any more expensive than Andorian makeup!