While I don't object to "beyond level 20" concepts, I would like to point out that DnD 5e (which this game is based on) doesn't have levels beyond 20. I'm not sure facing down a deity is the correct path, but I do think consortium with deities isn't a bad idea. Instead of human quest givers, you are doing the directed bidding of one or more deities. Perhaps in a bid to replace one of them. Something like the different deity factions are going to war and they're using your character to turn the tide.
Of course, given my theory on the game's story, we're already in the middle of a "god war" as it stands.