The Halo 2 Anniversary documentary 'Remaking the Legend' may be focused on Master Chief's second adventure but the video also has some tidbits about the upcoming Halo 5: Guardians.
In the documentary, which you can watch here, we see some images of Agent Locke, the deuteragonist of Guardians and main character of Halo Nightfall, loading what appears to be a battle rifle. We also see Locke with a Sangheili, possibly one of the Arbiter's followers. Finally, he's seen standing alone.
These images appear to be from cut scenes. It's been suggested that they may be from terminal footage, or feature in an extended cut scene in the Master Chief Collection. It's also possibly that we've seen our first sneak glimpse of a Halo 5 cut scene.
You can watch a short video discussing the three images of Locke below.
It's also been revealed that certain achievements in the Master Chief Collection will unlock armour in Halo 5: Guardians, and the game's multiplayer beta.
Getting the 'Your Journey Begins' achievement in the Collection will unlock the Mark 6 armour Gen 1 for Halo 5. Another set of armour, the Mark 6 (Scarred) Gen 1, is available once you unlock the 'Cryostation' achievement. If you get the 'Legend' achievement, which unlocks when you complete all four campaigns on Legendary difficulty, you'll get the Helioskrill achievement.
The video below shows you more about these armours and some of the others you can unlock.
Halo: The Master Chief Collection, including Halo 2 Anniversary, and Halo Nightfall are set for release on November 11th. Halo 5's beta being on December 27th, and Halo 5: Guardians is expected in late 2015.