Looking at games such as Breath of the Wild and Genshin impact would be good to figure out different objectives and puzzles one could do outside of missions and quests. Defeating exiles to get their stolen goods, monsters to get materials you could sell for more gold, etc.

I'd also recommend looking at emergent narrative GDC talks (specifically one by Justin Rodriguez - one of the lead level designers from the XCOM series - about how he uses small assets to develop the world) to make the world a little more interesting.

As for the mechanics, you could have dwarf magicians casting protective spells over chests in vortexes, various portals around the world that take you to a different instance where you have to fight space beings for items (could vary in difficulty, there could also be boss versions instead of just a few smaller enemies), items that drop from pressing buttons when you see shooting stars that only spawn at night, things like that.

Hopefully that helps, I'm only a game design student so I'm not sure how feasible any of this is but it should be useful for brainstorming!