Halo has so much lore and backstory that games could be set in and around any point in it's history. I feel though as if it would be executed better through the use of FPS.
Sadly, even though I liked Halo: wars...A LOT... it seemed like an experiment that did not work out as well as hypothesized. ODST is a great example of a great FPS Spin off.. It felt like Halo, but at the same time Marty's music and the fact that you were no longer a Spartan made it feel very different. These spin-offs can be executed wonderfully, but there is not much substance gameplay wise that you can give to an FPS Spin-off.