I think very few people have noticed that the player's HP bar already has a small decoration that is visible to other players. However, in its current form, it's barely noticeable, blends into the UI, and doesn't really give the impression that the player is playing on a specific world difficulty.
This is how they currently look depending on the selected difficulty:



I suggest giving the HP bar a unique design for each world difficulty. This would allow players to immediately recognize which difficulty someone is playing on without having to look closely at the tiny icon next to their name.



ZOS, please consider adding something like this. It would not only make the UI look more visually appealing and diverse, but also provide a clear way to showcase the achievements of players who choose to play on higher world difficulties.
P.S - Of course, these are just fan concept designs, but you get the idea of what I'm suggesting.