Since Halo is a FPS, it should always stick to the Linear-Style Campaign gameplay.  That's just my opinion, but I believe that the overall consensus is still for a Linear-Style play.  That is basically the linear level design and story, which I believe is proper for Halo.  It is fun to see in levels different situations and variations pertaining to enemy locations, weapons, and vehicles.  Keep the story the same, but change the situations in the campaign missions. 

---

I don't want to kill you, it's just that you're too ugly to let live!

-Sergeant Johnson