Judging by the ending of halo 4 ( both main story and spartant ops ) and the trailer of halo 5 i think that the chief has found out about the attempted assasination of Dr Hallsey and has gone against the UNSC ( other than the infinity and lasky because of his past with lasky ) and has gone searching for hallsey in hope that she can bring cortan back ( because she was the original creator of cortana. I also think that because of the covenant civil war the arbiter might come to the chief for help because the splinter covenant may have found another halo ring or a forerunner artifact and could activate it with the help of dr hallsey
343 said this was going to be a darker game so in my opinion this mean that we will see the chief slowly become mentally or emotionally unstable which could lead to the death of important characters
As for where the game will be set judging by the trailer for halo 5 it could be in 1 of 2 places
1. One of the moons of reach because hallsey wanted to return and the chief may start looking for her there ( the trailer shows a red planet in the distance which could be the destroyed reach )
2. The moon of the elite homeworld as hallsey could have been taken there and again the chief tracking her in hope of restoring cortana ( the elite homeworld is red )
All of this is based on facts and my personal opinon, not sure how many people will agree just thought i'd voice my opinion
