I'm sure one if not all of these has been mentioned at one time or another but posting anyway My apologies in advance to anyone if you posted these prior.
1. Private Arenas. Maybe a portal you can place in your home to get there. Maybe it could be a craftable item and only artisans can make. I'm sure crafters out there would love that.
2. Not just new spells but new spell types or different variations on current ones.
3. More flexable user interface. The only thing you can adjust is the chat box.
4. Ability hang certain items like wands (umbra sword, crulean sword) on your wall to display or place our mounts at home to roam around like we can with our pets.
5. Unlock more free content. The Free to Play model is catching on in the MMO world I think if you unlocked more content you might attract more players(Not that you're having a problem in that area). More players = Win!
6. Live events! When is Malistare going to get tired of us wizards beating him up and exact his revenge on us? Would be cool to see malistare pop up somewhere unexpected and start battling wizards at random on the street(I'm sure that would be difficult to pull off)
My apologies in advance to anyone if you posted these prior. 