In the Halo 5 trailer, we know that Master chief is hiding his identity, but we don't know from who.
And we know that Master Chief is face to face with a massive Forerunner, but who says that it's there to fight? Maybe Master Chief was looking for it in order to somehow save Cortana, and my evidence to that is how Master Chief holds Cortana's chip while the that Forerunner thing is rising from the sand. It also seems to me like Master Chief fought through a lot to get there because of his cracked visor, which probably has something to do with the Didact or maybe even the UNSC.
But this is all theory, and it looks like we'll be getting no information until E3 2014.
Tell me some of your theories. Thanks for reading.