Halo x The Survival Element of Minecraft / The open world style of Crysis.
It'd take place on one of the many magnificent forerunner structures. The one small reason that has me thinking this may be the direction they're taking is because the new Warthog (as advertised in the Forza 4 trailer) was given distinctive traits that would make it suitable for exploration and survival. Such as the built in radio, the tow cable and the attachable gas tanks.
So, my imagination built upon this idea and I decided that I'd really love there to be a central base in this open world that can be returned to for supplies, whilst also being upgraded with anything that you find whilst exploring. The upgrades would slowly bring the 'base' back to full functionality over the course of the game. A more fully upgraded base would provide you better equipment and vehicles to help you explore the structure more easily. It'd also better protect you from any hostiles (which vary in aggressiveness depending on the dynamic weather and night/day cycles). I can just imagine Halo 4 opening with the Chief being separated from the Forward Unto Dawn and then lost on the structure with only a Warthog. The player would then be forced to explore the area, locate the crash landed ship, explore it for supplies and then use the Warthog to tow it back to base.
Missions would be focusing on exploring areas of importance (control room) and 'side missions' would be focused on gathering supplies to help you. Both can be done at the player's leisure, with more and more areas being accessible as the game goes on. I also love the idea of taking my Warthog out to explore, crashing it beyond repair and then being lost somewhere on the structure as night approaches.
As to deliver some of the more classic set pieces, the first half should focus on making UNSC contact, the second on some other shit.
It'd be so good.