This is a great idea and I think it is something that Rockstar will 100% already have in mind for Red Dead Redemption 2. The Chiliad mystery was highly entertaining to follow and only added to the hours upon hours of gametime that you got out of GTA V. They have to include something similar in RDR2. Heck, just look at West World, that map engraved on the inside of one of the host's skulls was/is a pretty cool mystery.