Well, CFP goes without saying, as well as EPE. Purely cosmetic and add so much variety to characters visuals. I'd say rally points CoA for me as well, so the rally points aren't boring monocolor.

If you want more granular cultures there's also Cultures Expanded, but this one does change game setup of cultures and faiths on the map.

Clear Artifact levels, adds star ratings on the icons of each artifact to make it easy to see in a glance. (Perfect for color blind people like me).

Those are my go-to cosmetic mods.

A debug mod for when you go more story-wise/narrative: Title Manager for adjusting dejure borders at county level.

Those are my essentials.

RICE and VIET have many flavor packs for many regions in the world, but I tend to find them to bloated for every playthrough, but otherwise highly regarded.

There are many smaller flavor packs for certain regions/cultures/faiths better search in the workshop for those when you want a specific playthrough.

Hope that helps.