While I don’t want to go back to everything being like it used to be for class design, I do wish they’d return an element of “bring the class”.
I liked the feeling that when you grouped up, you felt more powerful and not just because you had an extra sword or fireball. Other classes brought buffs or debuffs that made you stronger. I liked when they brought unique things to the group that made you think “A Mage would be awesome right now for X”.
The balancing would come from making sure everybody gets something that unique and awesome, and not arbitrarily deciding that one class should do half the DPS of another like happens to some in Classic WoW.