Interestingly, when Halo was still due to come out on Mac, before the Xbox existed, these are the exact aspirations they had for Halo.
The halo was planned to be a persistent, open-world, where you could travel to any point. Any changes to the environment would remain if you returned at a later point. In addition, combat would consist of specialisation. You could become a pilot, or sniper, and join teams with differently skilled players - bear in mind this was 1999 when these concepts were pretty original.
Obviously for the time this was hugely ambitious. It's interesting that Bungie may finally be able to realise their dreams.