There are a couple of Romulans that could have worked, for me, the first one I'm thinking of is the Romulan who was trapped in that planet with Geordi. the survivor of the 2 man spy ship/scout ship that crashed. it would have been a nice throw back to have him show up, now much higher in rank, and no so distrusting of humans, but still fully Romulan. I think the internal conflict of having trusted a human, a blind human, to survive, against the "don't trust the Federation" party line in the Romulan Military.
the MAJOR character I would have loved to see would have been the return of Tomalok. yes, he was busy with Babylon 5 and I love his character there, but come on, he faced off against Picard enough in the early years that a return of that character would have had a strong reaction. Seeing Fleet Actions there Sisko, Ross, Martok and Tomalok planned attacks would have been a treasure..