My first thoughts after fighting Zhaitan was that until someone dredges up his body I don’t belive he is dead.
Then I got to thinking about Zhaitan and what he is made out of, we have all seen him in game or in trailers now and know that he is made up of other dragons. I don’t think it would be too much of a stretch to assume he survives in some form and could once we have moved on to the other dragons starts to consume them after they have been deffeated.
So after fighting all of the elder dragons big Z comes back, but with parts of all of the other dragons patchworked onto himself. It may seem a little cliche, but wouldn’t mind some sort of superdragon at the end or I could just be thinking too much into this.