If the rulesets for different campaigns that you can join in PvP are not limited to only different durations or level limits, but allow slight gameplay changes, I would like to see a campaign with locked-in first person camera. Absolutely optional - if you don't want to be limited to FPV or don't like it - you just choose another campaign.
Short Version Why: it's a similar thing to when Chivalry, DayZ or ArmA have servers with locked FPV. Same goes for flying sims and racing games like Forza, for the audience who prefer more immersion.
Long Version Why:
First person only would make positioning and combat awareness much more important, as circling around obstacles and getting a jump on your enemies would be a more viable tactic.
Otherwise I can always zoom out in third person and see my enemy around the corner or over the obstacle and know which side they are going to come out and try to ambush me.
Also I think first person in PvP will make it more immersive and somewhat change the dynamic of battle. When I play it by default, I zoom out all the way and see the battle as if I'm playing a Total War game, almost from a birds eye view. That removes me from the immediate intensity of battles and allows me more opportunity to see where I'm needed from the tactical perspective as if my character had a UAV drone giving them intel on the ground.
Playing in first person adds that disorientation that a soldier on a battlefield experiences and makes it much more personal, as your view is limited and you have to rely on tactical guessing and more immediate visual cues to figure out what is going on. It is more challenging for sure, but if everyone is playing like that on the given campaign it's not making it unfair to anyone.
I didn't play like that by default since I knew it would put me at a disadvantage against enemy players playing in third person, but sometimes I did switch to FPV to feel the exhilaration of combat from my character's eyes and it increased the amount of adrenaline quite a lot.
There is a thread for this not created by me at ESO Reddit and seems that there is quite a few people who are also interested in such an option.
Edit: changes the first sentence as it is not confirmed whether gameplay aspects would be part of the rulesets for campaigns.