Firstly, server world files are stored server side, if a group felt so inclined they could spend the time making a custom world by hand. It would be nice if there was a random generator option. Or a map maker.
Additionally I would like customisation in the aesthetics of armour, buildings and weapons which can be stored client side and implemented into a server that has say 'steam workshop items enabled'. That would thus allow players to not wear cookie cutter armour or cookie cutter buildings. Making the game into their own. How to define it? Weapon recipe is calculated by size and scale of the weapon. Gold inlay zweihander? Large recipe. Simple kopis with wood handle? Very small recipe. Same for buildings and armour. Recipes relative to their design, complexity and size.
Obviously the custom items and buildings would be much harder to implement, however custom or random generated worlds would not be. The potential is already there.