#4 A message written in Samoan
In my opinion, this is a very likely possibility indeed. Brock Lesnar has not been kind to his advocate, and Heyman may be considering a change in clientele. To win Reigns over, we heard how Heyman spoke to him in Samoan, before the betrayal took place. Maybe the message contains a message in Samoan, assuring Reigns that Heyman was on his side.
Not only could this message be kept a secret from Lesnar, because of the language barrier, it would provide reassurance to The Big Dog, and make him consider Paul Heyman's initial offer. It could also be a message from Reigns' own father, telling him to get together with Paul Heyman. As we know, Reigns is a family man to the fullest.
I genuinely do think that this letter angle will play out in some manner at SummerSlam. One has to just wait and watch it unfold.