Okay, so working with what we know:
Warning: It looks long and may discourage you from reading it all, but I think it’s worth having a look at it closely. These are all the pieces of factual info leading into Halo 5… all summarised together. And of course, let me know your theories, based on all these real hints and clues to the campaign, underneath.
End of Halo 4: Cortana dead; Didact supposedly defeated; Master Chief back on Earth; Humans now in line to hold the Mantle of Responsibility;
End of Spartan Ops: Dr. Halsey ‘captured’ by Storm Covenant/Jul 'Mdama, wants revenge against UNSC; Requiem destroyed; Two halves of the Janus Key, which holds the location of every Forerunner artifact ever constructed, held by both the UNSC and Storm Covenant.
Latest Escalation Issue: Mention of the New Colonial Alliance, supposedly an opponent to UNSC, consists of all species, arguably a remnant of the Insurrectionists (also, due to one of 'Mdama’s commanders moonlighting for them, suggests an alliance with the Storm Covenant); The existence of inter-seller worlds/colonies consisting of human-covenant alike; A single Flood infection-form on the abandoned UNSC Spirit of Fire.
Halo 2013 E3 Trailer: Master Chief working alone; Desert surroundings; Concealing identity with cloak; comes face-to-face with a large, eagle-like metallic creature (which we can presume is Forerunner); 117 supposedly looking for Cortana.
Halo 5 box-cover: ONI Agent Locke (male) on the front; Master Chief inverted underneath him; desert surroundings.
Halo 2014 E3 Trailer: Agent Locke watching clips on the Chief; He is advised by the Arbiter on how to find him; Arbiter hesitant on helping Agent Locke, but stresses the importance of finding the Chief- stating everyone’s lives are at stake; Arbiter states: ‘The seeds of our future are sown in his past’; Arbiter momentarily seen in the same room as Locke, but the reflection off Locke’s visor suggests the Arbiter was from-or has a relationship- with the desert-like surroundings from the window (or television) screen.
Nightfall: Prequel to next halo game; Follows the story of Marlowe, recently confirmed to be agent Locke;
Promotional Halo 5 Cover art: Desert-environment of (arguably) a city. Suggested to be Sanghelios, due to what looks to be large Elite statue holding Energy sword; Flood pods (arguably) flying above the landscape.
Books: I’ve only read The fall of Reach, the flood, and halfway through First Strike, so my knowledge of this as a viable source is limited. However, it’s fair to say that the UNSC and ONI share a bit of friction, and that the ONI is bent on finding information and guarding secrets.
Okay, I’m sure I’ve missed a few things here and there, so correct me where I fall short. But otherwise, I think this is a pretty good summary of all the factual, relevant pieces of information leading into the Halo 5 Campaign. Based on all these sources, I’d love to read some of your Halo 5 Campaign theories based on these facts.
Cheers