Well, like I was saying earlier, I think it'd be a more interesting product if they made it so that it was a game about Cortana reflecting upon the Chief's past post-Halo 3 while he's in stasis. Hence the "flashback" rumour. In that aspect, it could be like a part 3 to the Halo Legends Origin cartoon episodes, but as a game. That way they could say take the most important/good levels from Halo 1/2. Given Cortana's state of mind, they could take a few liberties with the gameplay aspect at the same time. And also given that it'd be from Cortana's perspective, they could skip Arbiter's levels or make new "speculation" levels of what occurred.