Honestly, I am a little annoyed by how badly they screwed up the Euron plot line when the book provided everything they needed to make the events they wanted work but not be ridiculous.
In the books he is addicted to shade of the evening (the blue/purple stuff the warlocks drink) and it is rumored that he has captured and enslaved several Warlocks allowing him to magically gain advantages in seamanship.
Just make that the reality for the show. Rather than make him some goofball with one-liners to show how "omg crazy" he is, just show those rumors being a reality. Show the captured Warlocks, show him using them along with blood magic/rituals to create unnaturally favorable storms/winds. Show him callously capturing and sacrificing innocents (women and children) for those rituals. It isn't that complicated.
That would show him as deeply and disturbingly evil (especially if he personally partakes in gruesome blood magic rituals). It would also allow him to foil Daenerys' and Tyrion's plans without making them look like strategic fools (especially Tyrion) which is out of character. Have some comment in a planning meeting about how they know his fleet is at X location so he can't possibly get to Y in time, so they make a strategic blunder by him showing up there when it should have been impossible. We the audience know how it happened so it feels logical, but at the same time it foils Tyrion without making him look uncharacteristically inept. It would also add to his mystique in the eyes of the Daenerys contingent, which can be used later in the plot.
I mean, ffs, I came up with that in like 5 minutes and it wasn't hard. It isn't perfect but it is better than what we got.