We've all seen the kick arse new trailer, even though, barely anything was shown, apart from the blue outline of the planet which is clearly reminiscent from the legendary ending of Halo 3, so prequel or not we know that in Halo 3's timeline it's fairly clear Chief is heading to the planet that is going to be in Halo : Reach.
Personally, I think Reach and the battle for reach has a huge structure that could easily be dived into through the medium of a video game. A future one wouldn't make much sense so a prequel would be fantastic.
I personally, am hoping for a class based Halo FPS, like the Battlefield series, but in Halo terms.