Support multiplayer in Adventure

E.g. the boss could be 2v1 or in the most extreme 4v1 (at first)

That should provide a fairly easy to estimate additional challenge that's also obvious to the player on encountering he might not be able to beat it with his current loadout

so there's some potential you can make a few "simple" quests that would grant your team additional AI for these boss battles (or via an item that's hard to charge up)

Granted the trope of "gathering allies" isn't that surprising but imo you would have a nice tool for a different form of grinding that you can use to stretch different parts of the main story to your liking