I really don't know about this. On the one hand it could be good but also I honestly think Halo 3 managed to finish everything up and tie up all the loose ends. The Covenant is defeated, the humans and Elites are allied together, the Flood is wiped out and the Halo rings are deactivated and the Ark has been destroyed.
Everything is finished, there is no substantial need for a continuation of that. Sure it was a little ambiguous when Masterchief says to Cortana:
"Wake me, when you need me."
And then we see their wreck of a ship heading towards an unknown but glowing planet, but maybe some stories work better with ambiguous endings. Would Inception have been nearly as effective if they made a second one all about whether or not Cobb was really in a dream or not? Would Watchmen have worked better if it had a sequel all about whether or not Rorshach's journal messed up Veidt's plans?
Sometimes you need an ending that leaves the audience just wondering what happened next, without any actual continuity.
All I'm saying is, Halo felt really done to me. The Halo series that I loved began with Combat Evolved and ended with Reach. And while I'd love to see more Spin offs set in the Halo universe (come on, game about the Covenant Elites) I don't see any need to 'continue' the story line. After all the tagline for Halo 3 was 'Finish the Fight' and at the end of it you do indeed 'Finish the Fight'.
All I'm saying is, I am unsure of how to feel about this new storyline.