We shouldn't forget in this period of transition that the technical brains behind Bungie is Jason Jones (not the guy from John's Stewart's The Daily Show). Historically, he has shown an inclination to not want to retread ideas endlessly. He created his own game Minotaur before joining Alexander Seropian at Bungie.
When they wanted to create a 3-D version of Minotaur, they apparently found the format lacking and so modified the ancient Greek location to a Mayan pyramid inhabited by a god-like space alien to create the game Pathways Into Darkness. I enjoyed that game, with its semi-RPG like structure a lot. After that, Jones started Marathon, a science fiction themed game that also formed the basis for much of Halo's structure and backstory. What's relevant to me is that Jason Jones was involved in a heavy engine rewrite for Marathon 2 that saw much improved full screen performance, but did not involve himself in the next sequel, Marathon .
Instead, he had already started following the thread of another project that interested him: Real time strategy in a 3-D environment with desformable terrain. This was quite an about face from Marathon, and was quite interesting in its own right. I am not sure how involved Jones was in the making of the sequels but it is clear that he again felt ready to move on and craft something new.
This turned into the project codenamed Blam!, which everyone now knows as Halo. As an outgrowth of ideas in Myth, Halo was originally slated to be an massively multiplayer game where players took on certain roles. The idea was for every gamer to find their niche, like warthog driver, pilot, or sniper.
From a development perspective, it looks like Jones was heavily involved in the first sequel to Halo, but again, I am not sure how involved he was in the next sequel.
For years now, I have been wondering what he is up to. I figured that Jones has followed his same pattern and has decided to follow up on other ideas that interest him. After ten years of Halo and the prospect of endless sequels churned out by Microsoft, I think many people are very interested in what Bungie will do next. Previous comments have speculated that the publishing announcement with Activision will mean that Bungie will have to squeeze out a product very quickly and that the quality may suffer for it. I would contend that they have probably been working on something for several years at this point (there is a joystiq article from 2006 that states that Jason Jones was working with Halo programming lead Chris Butcher on a secret project).
Here's hoping for something exciting.
And by the way, I wouldn't mind seeing something that revives Pathways Into Darkness or Myth again.