Actually, I just had another idea inspired by something someone else said.
In ESO, you could find random chests scattered about in the overworld. They almost never had anything good in them, perhaps a little gold, some outdated materia equivalent, random crafting materials, etc.
But even a small incentive like that had me exploring off the beaten track while doing quests, purely because I could. It's not even about the reward, but the little hit of dopamine that comes from finding a shiny thing.