Instead of making your own firefight maps why not just allow AI spawning in forge. That way you can do both, make firefight missions or even your own spartan ops. I am sure the new x box 720 will most likely be able to handle something like that, I am sure it is possible but it would also be nice to add stuff to forge like wild life. Say that the map you are building on alows you to spawn a certain creature that will attack you. It could be big like the beasts you fight in halo: reach or they could be small like the crawlers from halo 4. But you could build a map where it is blue vs red then these monsters show up and start killing everyone. I dont know about anyone else but I definately felt the adrenaline when I was fighting those monsters from halo: reach.