Proposition:
A new preference category: Dealbreakers.
Said category would list a bunch of Sim "traits". Not traditional traits...new "traits". Things like "Goth" or "Horse Girl" or "Victim Complex" or "Loud" or "Hyperfixated". Vague descriptors of "types of people". Each of these traits is purely imaginary- it has no actual in-game behaviors or interactions or special moodlets. It exists for the Dealbreaker system, which works as follows:
If you select Like on a Dealbreaker, it marks your Sim as that type of person.
If you select Dislike on a Dealbreaker, it is a dealbreaker for them.
If your Sim is marked as a type of person that another Sim has chosen as a dealbreaker, or you have marked a dealbreaker and you try to interact with a Sim featuring that trait, social interactions will nearly always fail.
"My Sim is a Horse Girl and he HATES Car Guys".
In this case if your Sim interacts with a "Car Guy" basically every social interaction just plain won't work. Car Guys are a dealbreaker for Hendlefrond Chesternut, self-declared Horse Girl, and so he cannot possibly ever get along with Dave Trucknuts, the self-declared Car Guy.
I think this would be a fun and cute little way to add more "personality" to your Sims without actually changing who they are. Self-declared "imaginary" aspects with one goal: To obliterate social interaction with people who just plain aren't into that kind of person at all.