Well we know he's going after Tyrion. He is leaving to get Cersei a "gift." What does she want more than anything? Tyrion's head. Tyrion also happens to be with the other Greyjoys in an area particularly conducive to an epic naval battle. The only question is whether he get's him or not, or who he kills on his way to him. My guess is that Euron dies at Dragonstone, but Dany loses too much of her fleet to move in on KL via sea.