Euron will use magic to control the dragons
On Reddit, there is basically an entire PhD seminar exploring the idea of Greater Magic on GOT -- that is, magic that's more powerful than the everyday magic practiced by wargs, pyromancers, and others. For the TL/DR crowd, it makes a really interesting case that Euron will use this Greater Magic to defeat essentially everyone. Now, the catch is that the theorizing Redditor is mostly citing the books, but we can take some of these ideas to work for the show as well.
The biggest of all of these is Euron's Dragonbinder horn, which is allegedly a magical horn that binds a dragon to the will of the owner of that horn. We haven't seen it on the show, but Euron has it in the books. Will he be able to blow said horn during a battle with Daenerys and take control of her dragons (or, at least, one of them)? Would this result in some dragon on dragon warfare and allow the theoretical Euron/Cersei empire to even the odds against Daenerys and crew? Who is willing to pay the iron price? Also, what is the iron price for a carton of eggs?