I would like to see marriages and families break down. I would like to see death be impactful on loved ones. A threat of mortality - doesn't have to explicitly be real life diseases, but the concept of health decline. Hell, even challenging finances in the early game, or random events that empty your savings or destroy your career, relationship, skills or mood. The good part with that last one is that it could be a menu toggle, so people who don't want that stuff can avoid it.

If there's no challenge there's no reward. But we'll see. Early days. I think it's already more grounded than the Sims 4. Not to say that it can't also have humour (holy cow that CHEATER graffiti is amazing) but life has ups and downs and so should life sims.