So, based on what we know I feel the need to talk about the story, and where I think it’s going.
Sources of this speculation are taken from Concept Art, Character Information, and somewhat from themes rising in the plot of Halo 4.
So let’s get started with what the Chief is doing.
Master Chief has left the UNSC for (according to Escalation) “unknown reasons”. Nobody knows his wherabouts. At this point based on concept art, and the revealed Halo 5: Guardians title image it’s safe to assume that the planet in the image, which appears to be an Elite controlled world and possibly Sanghelios, is one place Master Chief intends to go.
Based on rising themes in Halo 4’s Spartan Ops, as well as a running pattern throughout the series’ novels, the Office of Naval Intelligence represents an almost villainous side to the UNSC. Not only that, they are growing in influence and power as time moves on, especially now that they are in control of the Spartan IVs. This informations is what I believe to be the reason Master Chief left without telling anyone, and the reason he made himself untraceable. He is aware of ONI’s growing corruption of the UNSC, and wants to fight it.
As for why he winds up on this planet, there are two possible explanations: Either he is trying to track down the Arbiter, and gain an ally against ONI. Which I believe would be a safe bet for Master Chief, considering he honestly can’t trust anyone in the UNSC. Or he has gone in search of Dr. Halsey, and is trying to gather information on Jul 'Mdama’s whereabouts, and by extension hers.
With Jul 'Mdama and Dr. Halsey now working together, both bearing a level of hatred towards ONI (let’s keep in mind that Jul is not necessarily a bad guy, but he is aware of ONI’s threat to his race, and will do whatever it takes to stop them.). This could lead to an interesting alliance between Master Chief, and the Covenant Remnant. But I dare not speculate further other than to say that the direction I see this headed is UNSC civil war, with Elites on Master Chief’s side of the fight, and Spartan IVs on ONI’s. (I personally believe Spartan Thorne, and Captain Lasky will fall on Master Chief’s side as well, but I’m starting to stretch sources of speculation thin to go any farther).
Now for the Oni Agent in the Halo 5: Guardians title image. Seeing the mirrored reflection of said image, with Master Chief’s appearing distorted, I believe this Agent is tracking, and trying to find Master Chief. Because the Agent works directly for ONI, this leads me to believe that he’s either being sent to gather intel, and spy on the Chief, or he’s being sent to assassinate him. There’s a good chance this will end two ways, either the Agent will play the role of ‘main villain’ in the game, or he will find Master Chief, and after spending some time gathering intel on him, will realize Oni’s corruption and change sides. Once again speculating towards my eventual civil war theory.
I believe we will play some portions of the game as this character, I don’t know when he and Master Chief will meet, it could even be very near the beginning of the game, with a final showdown (if my first guess is true) not occurring until he identifies Master Chief as a threat to ONI, and determines the need to execute him. If the latter is true, a tense moment where Master Chief discovers his orders, and a suspenseful gun-to-head last minute decision stops the Agent from killing him, and he admits his loyalties have shifted could be an expected part of the game.
Final speculation on plot is that I believe the end of this game will mark the start of the UNSC civil war.
* This section is PURE guesswork *
As for who the Agent is, we already know it’s not a pre-existing character, but taking very close looks at his armour suggests he may not even be human underneath. There is no “undersuit” to his armour, all there appears to be is an underlay of more armour. It’s entirely possible this Oni Agent is an AI in a robotic body. What AI he is has obviously yet to be determined. But that is one theory I’ve built about the character.