Why would you even be thinking about an expansion before you get this horrible game fixed. I have spent more time trying to log in or getting kicked or not being able to play because of broken NPCs than I have actually played. I would rather have my money back and be poking myself in the eye.
Housing
Sailing
Open world PVP
A fix for the guild traders (relocate them all to capital cities only maybe)
Thieves guild
New zones
Do away with VR and Gold/Silver
Smoother combat system
A proper ranger/rogue type class or do away with classes altogether
Better looking armours
Hide shoulder/hip flappy things
Finally a character editor!
ZOS go take a tour of some of the things POE has done with regards to player bases. Then go have a look at what Warframe has done with Dojo's.
Implement.
Also of course all the other stuff as well but things like being able to display armor in the guild hall and so forth would be epic. If you cant also have personal housing then have a portal or something in the guild hall that allows you to teleport to a personal house or location. You could even ninja the Guild Wars style so you can invite others to see your armor sets and weapons.
The stuff that was supposed to be here last fall.
Imperial city
Full justice system
Thieves and dark brotherhood, with full quest lines. Not just, here you go join here, get armor, look a skill, and done.
All the new updated armor as seen before.
Oh wait, it said expansion. As in paid expansion?
All the stuff we are waiting for was supposed to be done while we paid a sub. So, I expect it to be an update, not an expansion.