Why not have two systems that work independently but also enhance one another? An attraction and a chemistry system.
Being attracted to certain attributes about someone doesn't guarantee there'll be chemistry between the two people. So an attraction system could work where you pick what your sim finds idea in a romantic partner (hair color, eye color, fitness, job status, wealth, occult...) or what they are repused by in another sim (smelly Sims, unemployed/employed...). This should cause there to be different levels of attraction; say you pick three attributes a sim finds attractive in another sim and three attributes they dislike in another sim that when added up would ultimately determine how attractive they find another sim.
A chemistry system would have more to do with how compatible two sims are with each other (things like trait and zodiac compatibility, and skills/hobbies). That way a sim can be attracted to another sim but not compatible with them because of their traits or zodiac sign.... Likewise a sim could not be attracted at all or even repused by another sim but get along great as friends. You could have true soulmates (both sims are attracted to one another and are completely compatible). Or two sims could have opposite traits but compatible zodiacs, creating the "odd couple" dynamic. It'll allow for more unique relationships dynamics to form more naturally in game!
Also unrequited love/crushes! ?
SimA is 100% attracted to SimB and they have great chemistry, the only problem is that SimB is not attracted to any attributes of SimA. They are friends but only SimA wants to be more. ?
I mean, we already have the relationship lables and a romance and friendship bar, all that's left are the tools for us to control and customize it per sim.
Did I mention it should be completely up to the player whether or not their sims have a preference? As in you, the player, can choose to completely opt-out on choosing what a sim is or isn't attracted to, or only select one or two options on one or both categories. Because it's really important to have options.
Attraction system determines how much a sims is romantically interested in another sim. (Sims are more likely to accept flirtations the more attracted they are to another sim.)
Chemistry system determines how compatible two sims are for a relationship/friendship. (The more chemistry the faster the friendship bar increases and the slower it depletes.)
Basically the Sims 2 turn on/turn off and chemistry system mixed with the Sims 3 attraction system.
Also only teen-elder should have an attraction preference but chemistry should affect all sims.
