It's a super secret... if they reveal any detail, other developers/publishers will of course jump on it at first glance. Out of all the developers... Bungie has proven itself to have one of the highest level of regard when it comes to their fan-base and their product. They capped off the trilogy with a definitive (yet satisfying) end, and while it did leave it open for more sequels... it wasn't as blatant as other games have done. They decided to move on and explore other projects instead of staying with something that was already successful... unless I'm completely wrong they're experiencing some kind of internal disruption within the team. But I doubt it. Patience. (: