With the Dothraki hoard, the Unsullied, and the Dragons, Dany is pretty much going to have undisputed control of the land and the sky. So I'm guessing Euron "gift" is going to be complete control of the sea. He's wants his vengeance against Yara and Theon for stealing his best ships and standing against him as king of the Iron Islands. I'm guessing there's some sort of Naval battle and he kills or both of them. So maybe his "gift" is going to be Yara's head.