I don't want any separate instances. Just the same open world Tamriel. PVE isn't affected anyway because they wouldn't turn on their hostile setting in the first place.
There's a hard enough time just getting enough players to load a BG, no need to make anything else we need to load into

It has been brought up before, and occasionally, a new thread pops up on the subject.

Zenimax almost added something similar to the Justice System but changed their mind. It's probably mostly because the player base balked at it. Sure, not everyone did, but more than enough did. I doubt it was hard to sway their decision since when asked if a PvP flag system would be implemented, Rich replied, "No, at the very early stage of creating the game, when it was a concept, we made a strong-willed decision."

I was getting my idea from old school Diablo 2. It was just a "hostile" button.

People would usually PvP just outside of towns.

I like the freedom we used to have in old games like that. These days developers just over think things too much. It didn't need a bunch of bells and whistles.
It's such a simple thing to implement.

Personally, I love the idea of flagging for pvp in all zones. that said, I dont think much of the population would participate, and youd just have a small handful of people actually looking for PVP and simply not finding any, most PVP'ers already go to PVP activities and most PVE'ers wont want the interruptions.

It would be really cool for more open world pvp though, Id love to have some sort of event that maybe could aid your alliance in cyrodiil. Just for examples sake, maybe its its a single control point that spawns in craglorn, and it needs 100 ticks of 1 alliance to 'win', similar to BG, but just 1 flag, or maybe a moving caravan, or maybe a dragon that needs to be beaten into submission and the alliance that deals the most damage gets a buff. It would be cool to see pvp branch out into the actual world and also provide an impact. maybe holding the control point gets a scoring bonus for the next hour. maybe capturing a caravan makes wandering merchants in cyrodiil of the winning alliance suddenly be raid bosses that bust walls down volendrung style. maybe beating up the dragon makes a dragon appear over the next cross swords on the map, or maybe over the winning alliances home zone, applying a debuff or damage ticks to all but the faction that won the event. Obviously these are just ideas and not fleshed out, balancing, exact numbers, mechanics etc etc. But stuff like this could breathe life into pvp and also provide incentives to both be outside cyrodiil, jump into cyrodiil afterward, and have pvp in fresh new places that might be unexpected.

all that said, im pretty sure the server hamsters would not survive any of this. so... pipe dreams lol