What do you want out of a Romance Expansion?

A chemistry system should to be divided into Social Chemistry and Romantic chemistry.
Friends and lovers. This is basically a mix of personality trait compatibility, interests, likes and dislikes, sentiments. and the friendship and romance levels.

And then there is the attraction system, which would be based on physical traits, grooming, and attire.
The combination of the three: social chemistry, romantic chemistry, and physical attraction should create quite the range of social relationship levels.

I've played with mods for this and I found that sometimes I would roll with what happened naturally when sims became attracted to one another and other times I manipulated the factors, so certain sims would be destined to be together. I agree with mightysprite. It would be great if NPCs could initiate interest in your sims, or propose marriage. There should also be several responses your sim could to another sim's advances.