When someone dies or bullets get shot at a wall, there is a script code that runs for a few seconds then goes away. When the code runs out so do the effects. All they would need to do is have the code stay in place so that dead bodies, blood splatter and bullet holes stay in place.
We might not 10% of what we want but even a few changes would be nice. I think by Halo 6 we will have the forge we all want. The technology is moving so fast that everything will probably be built in a cloud system, not on your console, and saved there as well. The game would then be rendered in real time and streamed to your system.