An open world PVP server only works with multiple factions. The factions of Eorzea are not at war like those of Azeroth, it would break their alliance to have everyone fighting.

A fourth faction for Ishgard would be a nice touch however. I hope there will be some thought of an open world battleground similar to Wintergrasp in WoW(http://wowwiki.wikia.com/wiki/Wintergrasp).