In the past proc sets were used just for the sake of damage, examples like Tremorscale and Viper Sting were common. As new sets were added to the game it became popular to build into a certain fantasy like an Ice mage for wardens and Fire mage for Dragonkights, players were using sets as an extension to their builds and it made builds feel more personal.
Unfortunately most sets were nerfed into the ground 3 months after their release and now theres an outright ban about to happen. This will not solve the balancing issues though, newer, stronger sets will be added to the game and more than half of the older sets won't be relevant.
Solution: Now that proc damage is streamlined, for an additional 5th piece bonus, add a 2% increase to that sets damage type per piece worn. Add more types of damages to the game and put them in sets that just deal physical/magical damage. In addition Zos could add one new skill to the undaunted skill line per 2 DLC dungeons, that would have a damage theme tied into it.
Moving to a damage type combat would make use of so many older sets and open the door for newer ideas like new Champion points and build themes. ESO is beginning to get way too big for its current balance formula, with the way things are now there will always be a few sets that are meta and the rest are simply non relevant.