[url=http://ca.ign.com/wikis/halo-5-guardians/Speculation]Direct link to original version.[/url]
[url=https://s3cmsassets.blob.core.windows.net/media/Default/web/news-and-blogs/screenshots/full-size/halo-5-guardians-visual-id-teaser-wallpaper-1920x1080-e6bb97b9563b44cbbb552bf8e3d2d9c6.jpg]I'll start with the logo-screenshot first.[/url]
Here we can see two Spartans. One that we're all too familiar with, Master Chief John-117 Spartan-II, and the other figure who we've never seen before. It's this new, second Spartan that interests me the most for right now.
Clad in steel-coloured armour, this Spartan has one very important trait, and no, it isn't the glowing blue visor (I think that could just be for dramatic effects since John's is also glowing a bit, more like shinning). If you look closely at the chest plate of the upper Spartan, who seems to be a Spartan IV but could be something entirely new (I'll get to that in a bit), [url=http://i.imgur.com/796YJyK.png][b]he[/b][/url] is donning the ONI symbol.
Remember back to the the early moments of Halo 4 when the opening cutscene played?
[b]Shadowed Man:[/b] Do you think the Spartan's lack of basic humanity helped?
[b]Halsey:[/b] What are you after? The others before you were Naval Intelligence, but you...you're something else.
[b]Shadowed Man:[/b] Records show Spartans routinely showed mildly sociopathic tendencies: Difficultly with socialisation—
[b]Halsey:[/b] The records show efficient behaviour operating in hazardous situations. I supplied the tools to maintain that efficiency.
[b]Shadowed Man:[/b] Do you believe that Master Chief succeeded because he was, at his core, broken?
[b]Halsey:[/b] What does John have to do with this? You want to replace him...
That last line is the important one. This cutscene takes place a while before they find John alive. They believe him to be dead. During this time, I believe that they did actually create someone to replace him. Not your typical Spartan IV, but a new breed of Spartan more similar to the Spartan-IIs.
This could mean a few things for our olive-clad hero. It could mean he has a new ally in the battlefield who can understand him far better than anyone else. It could mean that while he is out looking for the remnants of Cortana in Forerunner technology that this new Spartan will replace him in the war.
It could also mean that ONI thinks that John is a loose end who is also a liability. A lab rat that is no longer needed in the new age. This new Spartan very well could be John's greatest challenge.
[url=https://s3cmsassets.blob.core.windows.net/media/Default/web/news-and-blogs/article-body/graphics/halo-5-guardians-concept-art-ca11b3e478cb49959c1bc3f7bca3533f.jpg]Onto the planet.[/url]
Now, the planet we saw in the reveal trailer is widely believe to be the Lesser Ark (the Greater Ark was destroyed in Halo: Silentium while the Lesser Ark is still mostly intact). That planet vaguely shown in the above image or the lower concept are however, doesn't look the same to me.
I think this could very well be Sanghelios, the Sangheili homeworld. We believe the Arbiter from Halo 2 and Halo 3, Thel 'Vadam, will be making his return to the series very soon, and this could very well be why. It's also somewhat important to note that he is wearing his full armour in the above shot. This could mean he's on Sanghelios before (or after, but that's less likely) he's on the GA.
Maybe the new Spartan has been ordered to wipe out the Sangheili from their homeworld since Covenant Remnant are still attacking humans. It could be ONI's way of ending the situation once and for all.
Don't forget, this is all speculation on my part, nothing is confirmed.
Edit: Higgy Doc pointed something out. If you look at the new Spartan's helmet, he has some sort of emblem on it.
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It's like a glimpse into time.