I'm in the same boat as you, Noah. My server is extremely similar to yours. Each time I asked my players what they wanted to see it was either nothing or something that breaks the survival experience. (Like /fly) Because of that, we kind of just make up stuff as we go. We've done things like adding runescape style tasks that will take people to various places in the world. We've added hundreds of custom advancements. Dozens of custom titles that are obtained through grinding. Custom dungeon style boss fights, custom weapons, and armors.
TL;DR: Basically, you have to turn your PvE server into an RPG. Whether your players want full RPG or just bits and pieces, is up to them.
