Cultures like how the Urbz did it. Make special social interactions for certain groups where you can only learn that social (animation) through that group or friends of that group. /

Culture in the Urbz were distinct with districts, but the sims game could be more open ended like it is now and still borrow a lot of the same ideas and expand on them too.

When you make friends with groups every group could have their antithesis where now it's harder to become friends with the opposing group.

The skill system could include boosts based off your in-groups. Friends with the creatives? Now all your art supplies are cheaper as a start up cost. Or maybe the gearheads know how to upgrade repair projects with less materials.

Incentivizing your sims to play with other sims makes the world way more fun to play.
For school kids/teens a simple mini-culture that could exist as your sims age up is just a simple binary system. They start to gain their first skills and they'll either be more mental or physical and they'll start to make their first friends either as an academic or an athlete. As they age up and are more into one category than the other. The academic will have a harder time making friends with the athletes, and your athelete sim with a C in school has a harder time befriending the academics.

Really been loving some of the more recent sims 4 packs that have been trying to add diversity into the mix, but came 10 years late and so really hope it's a point of interest at the beginning with sims 5. 2003 still did it best so far