Riffing on your thoughts:

Killer blockchain app: Couldn't we just use blockchain to protect against video deepfakes? Hash every frame as the video is recording, then create an immutable ledger of frames. All "official" videos must be able to be verified with said ledger, and the ledger itself can only be verified with vendor-specific key licensed by a central authority. Perhaps two -- say, Apple has one for iPhone and they must verify as well.

This could certainly be the billion dollar opportunity blockchain fanatics have been looking for.