If I could make an expansion that would actually bring me back to wow hmm, thats a toughie. It would take alot to get me interested and enthralled again like i used to be.
I wouldnt really care what the theme was as long as it didnt have to do with fire or deserts. Fire has been done way too many times within the game already, and deserts are some of the most boring zones in any mmo. Its pretty obvious the next xpack will be a legion based one (which is interesting story wise, but mechanics are what will bring me back not the theme. I can experience the story through mmo champion for free).
Some features that could bring me back would be :
Raid scaling that has a minimum of 2 people so small guilds and friends can actually raid together. Make anything below 5 people not require a healer
Reintroduction of the vanilla wow pvp ranking system with faster bgs (10 min wsg, 15 min arathi basin etc)
Introduction of a 4th spec to every class that would increase the number of roles the class can fill (ex. tanking mages, healing warriors)
A out of dungeon path of gearing a character through hard group quests and challenging solo encounters, giving people the option to gear up through grouping with your own server.
reduced development of battlepets and more focus on dungeons. Im talking about barebones on the battlepets, no further development on it except adding new pets.