One of the (perhaps accidentally) genius parts of ESO is the combat system and animation cancelling/weaving. Few people master it without addons, accessing out of game video, or being coached by another player. For accessibility to advanced content it is one of the gateways that is not evident for many players.
Because the animations in game are often out of sync with the optimal timing for light/medium/heavy attack weaving with skills, it would benefit players if the UI somehow showed the GCD, gave feedback about whether, and what, the player "missed" in a GCD, and offered in-game help about how to approach weaving.
If this ruins the immersion, then tie the feature to trial dummies and their goal of learning and improving dps in trials.