There are so many armor sets in ESO at the moment, but from recommended and preferred build suggestions, it seems like only a handful is actually used.
I have the following suggestion:
Why not make Armor Ultimate Skills, similar to the new Weapon Ultimates, based on the combination of complete armor sets the player is wearing? Combinations of different complete 5 piece armor sets will enable a armor ultimate, ie a complete Sunderflame set and complete Way of the Air will give the player a "Fiery Wind" armor ultimate.
I think this could have a bigger impact in ESO than Weapon Ultimates, as different armor ultimates will come from matching different complete sets. I reckon this could encourage players to start using less popular sets, and will encourage more variety in builds.