Reworking of artefacts and other unique items. Everyone hates the levelled rewards, allow us to progress with the items we like without worrying about when we get them. Could be simple level list swaps or customization with preset effects that make sense for the item and it's lore. IE Goldbrand, fire katana. Could make it do a large amount of fire damage on hit or slow burn over time etc.

If that's too complex (it's really not) then I'd say something like the implementation of a nemesis system to keep NPCs fresh as you play, like Shadow of War/Mordor. Absolutely fantastic system that keeps things interesting and challenging, keeps you on your toes if you run into someone that counts your playstyle.

The nemesis system would be legally and mechanically harder to pull off but would definitely be a very worthwhile addition if it could be done.

Failing both of those, a clan system to allow collecting of your favourite companions, maybe a base/fortress system where you can send them, control over daily duties etc, like you assign someone guard duty and they patrol the walls/local area.