Simply put 343 are rookies and are never innovating they just simply copy what's popular.
Treyarch however knows how to make an interesting and well acted story. They are the kings of co-op with their zombies. If they were to be given free reign firefight or spartan ops would be a lot more fun and challenging. 343 gives us a paltry amount of gametypes with no fun party games while Treyarch gives us so many gametypes and different modes so the game never gets stale. And Treyarch games well too, which means more money for future sequels so Halo could be improved further.
TLDR version. Give Halo to these guys, they are proven and can handle themselves.