Let's forget for a second that XIV playerbase is massively PvE centric, and how open PVP goes against the very lore of the game and let's imagine for a second what's going to happen on pvp realms.

What reasons are there to go outside of cities?

MSQ - do you really, actually, seriously, unironically want to be ganked by a random NIN over and over and over and over while you're stuck in "wait" animation during MSQ? Or on a way to next NPC? Good luck.

Fates - depending on how important they are people are either going to do them in big groups without engaging in PvP, or kill people on sight because killing lvl 80 synced to lvl 50 is fun and easy.

Hunts - Early on in the game two or three big FC's will farm them while killing everyone else on sight. Later on hunt trains will resume.

Gathering - Every timed high level resource will be camped by massive FC's. And if you're not part of them, you will be killed. Since owning high level materials means owning the market, those spawns will be either highly contested between big FC's or shared if they are allied to each other.

Since this game doesn't have farming spots, big open world bosses, or some kind of territory control, fight for resources and random ganking will be the only reason to open world PvP. I have no idea why anyone would want that.