Estinien would be high on my list if that were the case. If only so I can swoon at his heavenly voice.
But, I think having characters unique to the romance quests would be easier in that you being romantic with an MSQ character may hinder the narrative later on. I mean, imagine if you were dating a Scion and 2.55 happened. I'd be a bit more bent on getting my lover back ASAP, and anyone with Haurchefant or Ysayle would lose their minds.
Having NPCs outside the main scenario would also be a great opportunity to expand on lore, especially for the different race's lifestyles and backgrounds.