You know what might get people to flag for PVP? Make it so flagging for PVP in multiplayer in adventure or PVP zones accrues a set number of Frequent PVP points per hour. These points can be accumulated and then traded for items in a special PVP tab in the add-on store. Items that you can't get anywhere else. Items like ice coconuts to go with the ice horse head, or a pet snake. Or any of a range of items that do not give any in game advantage.