When you level up a class and learn something, you should be able to use that on any other class.
Maybe to prevent over-levelling they should put in some sort of fatigue system so your exp earned will drop as you continue to level up.
To enrich the overworld, seamless zones without zone lines, transitioning from one part to the next by just going there. See something interesting in the background? You can go there.
To keep said overworld interesting it'll have to be dangerous. Slap some level 99 mobs in places you want players to tread carefully.
Dungeons would need some work. Branching paths, multiple optional bosses, and maybe a time trail to incorporate better rewards at the end.
Tell the story through full motion capture cutscenes, none of this emote and talk and walk away stuff we got now.
Also let's give the player some weight behind their motions and movement. None of this stopping on a dime and flipping around like it's nothing.
Crank up the graphics, I want to see a flower pot have as many polygons as a player model.