Oh man, I have an idea for a 5E character I've wanted to play for a while now. A fallen Cleric of Pelor who buys into the Burning Hate Heresy and is now a Celestial Warlock. His patron claims to be an angel who has also discovered the truth and needs a mortal agent to help expose Pelor and eventually take him down.
Whether or not any of it is true (the Heresy, his patron's identity, or any of the rest) I'd leave up to the DM.
Bonus points if it takes place in Matt Mercer's campaign setting. Haven't you ever wondered why there are two sun gods? WAKE UP, SHEEPLE! (Yes, I know, I know. It's because they switched from the Pathfinder pantheon to the Standard Issue D&D pantheon when they changed from a private home game to a streaming show, but they didn't want to change Ashley's character's god because that would just be a weird and awkward retcon. But just leaving it at that is not how crackpot headcanon theories work, you see.)
The other major headcanon I hold unwavering loyalty to is the Great Modron Marketing Survey.