A huge area where its open-pvp were guilds can take over villages (maybe castles) and fight for control over them or you can simply party up.
More over this shouldn't be faction based but rather Guild based when a Guild has taken over a village or castle it becomes something like a base where they can build turrets that fire arrows to defend off any intruders such as People parties that are hunting down other people.
There should also be mines and dungeons were Guilds can mine/loot the dungeons and travel it back to there village to store it. They then have to take that stored loot back to the city to sell it (these materials should be more expensive then on the Pve grounds). While these materials are transported people can try to steal them by killing the travelers of that guild or mine them secretly and hope they don't get caught by a huge number.
What do you think about the idea?