I would want a system that works both with shallow things like hair color or eye color, but also with things such as traits, jobs, hobbies, occult, likes and dislikes. I wold build it on the basis of likes and dislikes, so my dislikes would be things that my sim are unattracted to (and make relationship building harder) and my likes would be things that my sim are attracted to (making relationship building easier and autonomic romance a thing).
My biggest wish though, and I know this won't happen in sims 4 but I still have hopes for sims 5, is that relationship building was one sided. Just because my sim is in love with someone doesn't mean that they are in love too. Having crushes or unrequited romance would be fantastic gameplay.
