Instead of a colored circle why not have a circle themed for each class at least. The elements are already there.
Sorc-blue/lightning
warden-green/nature
dragonknight-fire/earth
nightblade-I don't play nightblade
templar-holy/light
necromancer-bones
The effects would pop up in your circle, for example if I'm a sorc aiming my liquid lightning it could have a circle with some animated lightning bolts in the middle.
The easiest fix is just have a animated preview of the ability. Add a option to enable/disable so people that want optimal performance can. But personally I don't mind losing something like %0.5 of my performance if I can have better ability circles.
For bosses it's also relatively easy. Each theme would be carried over and could also have the whole "ability preview" to make it clearer to see. Again add the option to disable it and have basic colors for people.
ex: if a knockback Aoe is about to be put down replace the circle with smoke there or a explosion happening on the ground. WoW has animated circles for bosses so I don't see it being that hard to implement. ZOS has made tons of money from the lootboxes and crownstore, so there isn't an excuse of "it costs too much money" and you can't say it would take too much time because microsoft owns the company so they most likely, if they havnt already, hired more devs.
I can't find a excuse not to add it because even if you hate the change I think it should be completely optional.