It would be pretty cool if you could make ai's in forge, like a Moa, or even fish, or birds. Also, you should be able to build more natural objects (apart from rocks!) like trees, or sand, grass, plants, etc... Also, there should be a special firefight mode where you can fight the flood! I loved the armour customization in reach, but I think halo 4 should take it a step further, like having an option to customise your feet? Or maybe even customise your height/weight, to really make your spartan your own! That's all I can think of at the moment...