Sourced from other forum posts as well as my own nostalgic memories of other games. Aside from the obvious class balancing (*cough cough* Jesus beams *cough cough*) and bug fixing...

1. Secondary PvP system
-GW2 5v5 system
Guild Wars 2 had both an open world and a 5v5 system. I think 5v5s would be amazing in this game, as well as open it up to more competitive play and more team strategy.

2. Magicka based melee weapons (bound weapons)
Stamina weapons: dual wield, two handed, bows, sword n' board
Magicka weapons: resto staff, destro staff
... c'mon, we need melee magicka weapons.

5. More side content
GW2 jump quests, points of interest, dynamic events, maplestory party quests, Witcher 3 bars w/ gwent minigame
Quests and dungeons are nice and all, but... anyone else have nostalgic memories of jump quests and Kerning City party quests, and anyways, the ESO world could use a bit more STUFF to do outside of questing and dungeoning.

4. Color enhancement
Witcher 3 color palette > ESO's 50 shades of gray

5. More social
Maplestory is a wonderful example of a social game. Guild quests and rankings, global events and competitions, as well as maybe some more ways players can compete with others outside of PvP?

7. Class change
Kinda personal because I got a guy up to level 50 and want to change his class and I REAAALLY dont want to farm on a new guy for the level... I'd pay like 20 bucks for it... I mean seriously, there are RACE changes! What makes more sense irl: Changing your race or learning a new skillset? Exactly.