I agree with the dead body's, ammo clips, blood stains and some of the environmental stuff, but mostly id like to be able to create night and day scenarios, as well as placing all types of structures, not just forunner. Id love it if they added more decorative things such as UNSC logo's for walls, and other stuff similar to that. Being able to place roads and A.I characters and the ability to change how they act in the forge world would be cool. (such as making them defend against flood, or to go out on patrol.
However this would require it to be on a seperate disc.