The ideas here are great and would definitely make romance more interesting in-game. Currently, it seems that if you carry out enough romantic and friendly interactions, you can force any pair of sims to partner off, regardless of traits, but that makes for some boring gameplay. Some might take longer than others, but if you're determined, you can have them meet and marry within the day. Having said that, here are some ideas I have that I think will make romance a bit more interesting.
Unrequited love - this one will make stories very interesting. You can have a sim in love with another, but they don't reciprocate those feelings, or are completely unaware of them, even though they're friends. Of course, this does require some sort of passive attraction system, because currently you can only have romantic interest if you actually carry out romantic interactions with them - the interest is created by us, the players. I hate that the relationship bars are always symmetrical. Some sims should be more/less receptive to certain types of sims.
Gender preference - it's great that you can adjust the gender options for sims as you see fit, but I also think there should be a preference system, where you can decide if the sim in question is attracted to only males, females or both. Some sims would definitely be straight or gay, but there are others that don't mind being with either. CAS would be a good place to add it in, right there with the gender customization options.
Faithful trait - there's already the non-commital trait, which means that those sims don't like being in the same relationship or job for too long and there's also the player trait you can earn from the serial romantic aspiration, so you can date half the town and nobody will care, but a faithful trait would be good too. It could be a reward store trait, since the soulmate aspiration already has a bonus trait attached to it. Faithful sims would be a lot less receptive to romantic interactions from other sims who they aren't in a relationship with and it would be a lot harder to convince them to leave their partners. Maybe they can even get angry at other sims who are persistent, once they find out that they aren't available.