I just hope there would be a game-play mechanic that would make it a bit more challenging to flirt up a Sim. Right now it is possible to meet and marry a sim within a sim day - and then rinse and repeat.
It would be nice to have turn-ons and turn-offs to regulate autonomous flirting (if the sim is attracted to brunettes then they autonomously flirt with brunettes)..
but still I would like to be able to influence the sims to overcome some initial turnoffs and maybe fall in love with some gameplay (when their friendship gets high enough and they have common interests and skills, the chances of attraction rises regardless of looks etc)
and stemming from that - asymmetrical relationships would be lovely - a bearded sim likes a brunette sim, that does not fancy beards etc.
however, there should be some dislikes that are extra hard to override (or impossible) - when I've set the sim to dislike infidelity then a married sim flirting them up would create distrust (perhaps not instantly but after asking "are you single?") and make it near impossible to become friends not to mention gaining any romantic relationships after the fact.
So I guess I'd like to see a likes/dislikes or turn-ons/turn-offs system for asymmetric relationships with an added layer of red-flags or super-no-nos system on top.