I’ve posted this topic in the Universe section of the forums already, but I’ll post it here too since it’s relevant to both.
There is a lot of speculation that in Halo 5, John will go rogue and hide from the UNSC. I have a different theory. It’s been more or less confirmed that John has a geas. I think the imprinted personality/personalities inside John will take over, at least temporarily.
What if John starts seeing things: images from the past? It’ll get worse and worse, and eventually, John may go a little crazy from the experience and the UNSC will mark him as insane. The crack on his visor could be a battle scar from him trying to escape. The marines that attempt to detain him could appear to be Prometheans or Flood forms because of his geas, and he kills them (the story is supposed to get darker).
His geas could lead him to an area or object of significance: an ancient human superweapon, a Precursor/Flood research hub, etc. As for what happens after, I can’t really guess.
Of course, this is pure speculation, and all of it could be wrong.