So I think its obvious knowone outside of 343 really knows what’s going to happen at the end of Halo 5, but my theory is that during the course of the game Chief and Locke will battle it out, it will end in a draw when the Didact returns and they join sides to fight him, they end up defeating him and with his last breath proclaims “if I die, so does humanity” Chief then proceeds to kill him in some way. There is then a cinematic showing there victory and the destruction that insued, and then finally at the end the camera cuts to a part of the galaxy far away, be it another planet or halo ring, where the didact put in place a system that if he died would allow the release of the flood from containment vessels! It would then cut to black.
This would then lead to the flood being the primary enemy in Halo 6.
Therefore I predict that the flood will not be in halo 5 but will be introduced in halo 5 and then an enemy once again in halo 6.
If I’m right, sorry for spoilers.