The proportion of dungeons where people no longer (need) to play mechanics is way too high due to power creep over the years. That's a huge amount of content people can clear without even knowing that there are mechs for certain (actually I think most) boss fights. Everyone these days just burns through, but the mechs are (supposed?) to be the interesting part, and the piece that differentiates one boss from another.
All base game dungeons would benefit from a difficulty above veteran, I want to be able to visit those and actually play the mechs to keep it interesting. Even a lot of DLC dungeons it's the same scenario. Maybe some of the very hardest DLC could benefit from a slight nerf to "veteran" difficulty but then the "new" lets call it "Advanced" difficulty gets buffed slightly compared to today.
What would the rewards be? I guess the easy option would be for set pieces to drop in Legendary quality but that's a bit of a cop-out in my opinion. (I semi-suspected the Jewellery changes for platings could be in preparation for something like this).
Lets consider one of the "features" that perhaps unintentionally makes ESO builds interesting: the fact that you wear 12 pieces. As 12 is divisible by 2/3/4/(5)+2 there are a huge number of combinations, BUT, sets with 4 pieces are basically non-existent in game. If 4-piece sets were much more prevalent, imagine the combinations (in addition to the above):
5+4+2+Mythic
4+4+4
So my preferred rewards, if an additional difficulty was added would be:
each (current) 5 piece set drops as a 4 piece set (the 5 piece bonus applying on the 4th piece) on "Advanced" difficulty. Put "Perfected" on the front of the name if you like.
There would be so much re-playability for existing content with that change, as an avid stickerbook collector, I'd rejoice. As someone who wants to engage with the content rather than just mindlessly burn bosses, I'd rejoice. Not to mention all the fun I'd have obtaining new achivs at a difficulty that is at least somewhat of a challenge.