Perhaps you could put in two factions you could be able to join but is not necessary , like the "good" guys and the "bad" guys. So if you choose one of those factions there is constantly Open world PVP between eachother, The good can always attack the bad guys and vica versa. let's say this feature would be only for more experienced players since you don't want new players to accidently choose an faction and then get attacked the whole time as they're grinding trough the storyline and levels.
For more experienced players this could be a cool feature and brings more "risk" and environment to the gameplay.
Some small rewards could come allong with it like cosmetic gear, titels etc... Just imagine if you're training somewhere and the enemy faction comes allong and starts to attack you, you could write towards your faction chat for help and your faction comes over to help you and a huge battles randomly appears. Huge PVP events could come with it too to take over "Towns" that are controlled between those two factions etc...
Also an toggle to put your PVP on and and off perhaps.
What do you guys think and are feel free to say and share your own idea's for even more content with this idea! :)