Well, I hear that they're actually making a halo 4. I also heard it isn't actually bungie making it this time. I read in a game informer magazine that they actually had a halo 4 video prepared for this years E3, but the very week E3 took place microsoft pulled the plug on it.
according to them they want to make an entire event just for the premiere of the game.
As for not being able to top halo 3...
First off I'd like to say that of course prefer halo 3 over Call of duty 4 any day. Halo is the the foundation of xbox!
But there are things in halo 3 that could be improved.. primarily forge. Not that I don't like making floating boxes all over the place, but they should at least make it easier to place up walls and fence walls. especially window panels, those things frustrate me so much trying to set them up.
You should also be able to have a forge level like foundary, but bigger. Also, a few things I didn't understand was why you couldn't
a: make a shade turret (the purple circular turrets in campaign)
or
b: make those floaty purple outpost things. (you know, the ones with the giant gravelift at the bottom that you ride up that has the turrets)
I also think that in forge you should be able to have some kind of lay out of squares so when you want to make an amazing course on foundary, you have a way to set up your boxes perfectly even.
Also, walls, fences, and single/double boxes should all be the same height.
Also, the story leaves off (if you've completed the game on legendary) of masterchief falling asleep in a pod and him and cortana float through space towards an unidentified planet. (but of course they are releasing new campaign missions soon that will probably have to do with this)
and maybe they could increase the online party size? like 20 people per team matches. that would be epic.
well.... main point : there's always room for improvement.
