There are plenty of activities in ESO, and still too many of players who "enjoy" spoiling other's fun by performing some activities in places designed for something else...
My 2 cents:
1. Overusing of skills/mementos. I suppose not many of us like -when doing bank activities or trading/crafting and so on- someone at our back is casting shards every second until magicka depletes. or popping own familiar to pulse endlessly. or running around bank casting vigor and endless hail. The same with mementos -being mudballed once might be fun. but every 5 seconds for 5 minutes... not much.
My proposal is: add one more duty (besides watching folks trying to perform activities connected to legendermain/thieves guild/dark brotherhood) to City/Bank/Castle Guards. Casting the same skill/memento for 7th time within one minute (or less) triggers nearest guard attention and caster is charged for 500g. Clemency is not involved, player accused for trolling may only pay bounty (and eventually loose stolen items, if in bag) or try to escape (bounty is increased in usual way). Recidivism (going back to trolling activities after paying bounty) within 10 min increases next bounty for 100%.
2. Dueling in City areas. There are nice place designed especially for dueling purpose, near Bonesnap Ruins in Stormhaven. Noone is using it, probably because of 2 reasons: there are no wayshrine inside and as noone is waiting here for duels, much easier is to find potential opponents in towns.
Or (PC/EU) at Bergama wayshrine in Alikr. Dueling in this place disturbs noone. Who wants, may stay and watch/take duels, sometimes with the participation of known youtubers; if not, may go to own activities. But duel (sometimes even 3 at the same time) just several steps from traders at Rawlkha or Mournhold, when you want to do some shopping in peace...
My proposal: add a wayshrine in the middle of Hammerdeath Arena. Might be "dungeon type" - if an duel invitation in city area is accepted, players are "grupped" and relocated to this WS, then ready check is performed. Duel countdown starts when both players confirmed RC (this will prevent killing opponent being on loadscreen status). After duel is finished, both receive message screen with 2 options: another duel (revenge)/return to previous Zone