Basic idea: Reimagine specific classes with purchasable animation packs. Basically you change nothing mechanically about the class, but you can buy a pack that changes the animations so that visually you reimagine the theme and allow players a new customization for their character.
Meant for your Dragon Knight to be more of a holy crusader? No problem! Pick up the Holy Knight pack and now all that fiery, smokey, earth themed stuff visually has golden glows and angelic visuals.
Did you always want to be a necromancer? Now you can! Pick up the necromancer pack and all your sorcerers casting animations appear to be tapping into the dark arts of necromancy, your pets will even be reskinned to be undead themed, though nothing has actually changed in what the abilities and pets do.
Did you mean for your Nightblade wood-elf to be less nefarious and more an elusive guardian of the forest? Then you should definitely pick up the Forest Sentry pack and strike out at the foes of the nature from the shadows as each of the animations are designed for that theme.
You get the idea. Zero game balance head aches, and players feel like a whole new class that they always wanted ZOS to add. This would make huge money in the crown store and it would be something gamers bragged about all the time to their non-ESO friends.