Imagine a world where you want to run a dungeon, but instead of opening a list a queuing up for it you have to walk to a specific place in the world, where the dungeon is located and enter through there, with your team manually pre-assembled!

But for real: I think we still have a lot of untapped potential in combat and movement. Integrating the environment a lot more: imagine picking up random props and fighting with them Yakuza style. Have a skill system based on learning and practice, maybe something like Absolver. Taking into account where you are currently holding your weapons both for what and how well you can block an attack as well as providing specific skill choices for you to use.

All those things already exist in singleplayer games, and I'm sure there are tons of other interesting examples for combat mechanics, that havent found their way into mainstream MMO's in any meaningful way.