Dyes. And materials required for each one different/rare.

[The Time Traveler]

-Add as rng/area/event* specific drops.

-Since they arent important to gameplay and would give people (like me) a reason to venture out into the biomes even after all else has been done.
*Could even add random event spawns that tie into these drops. Keep them random tho( like an invisible check each day would decide these and where or even make based off moon phases?). Makes the world feel more alive and unpredictable.

-Make ANYTHING dyeable. Building pieces and gear and weapons, even your pets.

-Make alchemy used to break down a discovered bottle of dye to learn its recipe.

[armageddon]

I think it would make sense to use dyes for armor/clothing and heck even hair. The vikings were known dye their hair.

[jonnin]

its in place for banners already, sort of, so could be done.