Here's your enigma for today: 343 Industries is in the market for a software development engineer to help "drive the future of Halo", a process that involves work with "external partners". Another hint, perhaps, that talk of a third-party developed Halo 2 Anniversary reboot is more than talk?
The right candidate will help 343 with "the challenging task of ensuring our external partners are able to turn world class games into industry movers". He or she "will be able to support external partners using a mature pipeline, drive new techniques and algorithms, take a lead on performance tuning issues and bringing passion and new ideas to the table. They will also be approachable and able to work with closely with artists and designers to understand requirements and solve problems."
Applicants should have "recently shipped a AAA Xbox One title", and will be expected to "get up to speed quickly on [an] existing codebase already in production". This suggests - and I'm stretching here, no doubt - that 343's mostly unrevealed Halo Xbox One will be the technological basis for any hypothetical parallel projects.
Halo Anniversary co-developers Saber Interactive and Certain Affinity are tipped to be working on Halo 2 Anniversary by a number of sources. Microsoft has stated that these and other reports are "based solely on speculation", which isn't quite the same thing as saying that they're false. "Other than confirming that your Halo journey on Xbox One begins in 2014, we have made no announcements related to specific titles," adds the statement in question.
Halo 2 Anniversary rumours aside, he mention of external partners could also allude to a project at Vanguard, co-developer of the moderately well-received Halo: Spartan Assault. It could also refer to an externally developed DLC pack for Halo Xbox One, rather than a full game. Your guess is as good as mine, frankly.
343 is known to be working on at least one game that isn't Halo Xbox One or the forthcoming Halo TV series. We'll hear more at E3, hopefully. In the meantime, you may wish to read Aoife's report on how 343 has staffed up for its Xbox One debut.