I feel that having an environmental aspect to forge would be a tremendous step for forge. Of there are weather patterns or any thing weather related, then the area of terrian should act as an environment should.
Mechanics: bring aesethic pieces to life. Make real action doors that operate on a open and close system. Allow the doors to be destructable by vehicles and weapons on course. Make covenant and odst drop pods(conevant pods seen in Halo Reach on The Long Night of Solace)
Maybe even for fun of it all create an alarm system for buildings. For instance, major clans in Halo like to make, what they call, HQ's which arr basically clan bases.
For blocks and pieces, you should make them have different themes( covy,human,forunner. Etc.) allow people to change colors of the pieces.
I dont think wd should even have a block format or use blocks as a building ground. Allow forgers to make maps as a game dezigner would. But just with a blank map with pieces. Also allow any piece of the map to be edited, changed, or deleted. Allow forgers to go to almost every part of the map.
343, if you really want to make Halo 4 good, with forge complimenting custom games, take a look at all the makes created by the communitt. Take the time to research. Go to forgehub.com for instance and look at the variety of maps they have. Be apart of the community!