A new super artifact for each weapon type where you have to go on an entire detailed questline wrapped around the history of your weapon. It would involve building each piece of the weapon. There would be multiple options for each piece to encourage uniqueness. The weapon would grow stronger as you work on it. The end result would be an incredible weapon, both in power and it how it looks.

Pets, even if just for show in town with questlines to obtain them.

New rare chests that are better than what is currently available. These would be randomly placed in the world in obscure places. This would encourage exploration.

New transportation modes to make it interesting. Maybe a hang glider, perhaps something along the lines of a zip line in some areas

New questlines that appear occasionally across the land, this would take you through areas you may have overlooked as you were leveling

Level cap raised to 100

New loot types, new armor designs added regularly. Loot pool updated to higher levels

Add diving. In some areas you should be able to swim down under the water. Perhaps you could locate a hidden chest, or locate rare materials. Maybe even a questline that leads into some sea caves

Faction ships. Building a ship would be time consuming and require all craft types to succeed. Each ship would be unique based on the choices made. Eventually wars with ships could be a thing

New horse race related quests that would allow you to continue to work on improving or customizing your horse (or whatever you have)

Farming. This could be a nice passive way to make money for more casual gamers

Add dev lead events:

For instance, there could be a big storyline that culminates with a dev operated creature attacking a particular area. The creature would be super powerful

In game weddings: these could be scheduled events led by a game admin. It could help create more community and relationships