Sacrifices to the Drowned God were apparently made by slitting the throats of victims and tossing them into the sea. This reputedly made the Drowned God stronger. Rhaegal certainly had his throat slashed and he plunged into the sea. What if the Drowned God considered this a sacrifice and brings Rhaegal back on behalf of Yara, not only to assist her but to punish Euron, who shows contempt and lack of faith towards the Drowned God. Even Euron's words, that he saw Rhaegal fall into the sea with his own eyes shows that he doesn't even give credence, nor consider the implications of what he witnessed and the potential.
Maybe a crack pot theory but there is this from the Lore: "What is dead may never die" is a common saying in the religion of the Drowned God on the Iron Islands. The answer to the phrase is "But rises again harder and stronger." In the episode the phrase is begun by Theon Greyjoy during his baptism and completed by the Drowned Priest conducting the ceremony.
Probably just wishful thinking on my part. :/
