I dont really think zhaitan was killed at the end of guild wars 2 campaign for a couple reasons. First I believe that zhaitan is a collective or hive mind if u will. I think that zhaitan can rise again from his undead minions to exact his revenge. Think about it in the campaign u fight enemies known as zhaitan's mouth and his eye. When u kill them zhaitan feels pain. Why is this? Then there's the fact that zhaitan receives the knowledge of the enemies he kills. To me this seems like the undead is a single mind so I think for a future expansion we go to another continent where zhaitan has been rebuilding him self since his fall in orr. This would be really cool because it would bring back a really cool villain who's boss battle was underwhelming and give him a chance to show his full wrath maybe we have to get past the sea dragon before we get to the other continent maybe where stranded on the continent because of the sea dragon. Picture this we go out to fight the sea dragon when we are attacked by his minions then the sea dragon comes from the depths and wrecks our ship we get stranded on a strange new world that looks alright at first but as we go further inland we are met with undead resistance. We later find out that zhaitan has been rebuilding himself since our last battle.