So I was thinking new Items have been added to forge already so I know this is possible but the thing is what Halo 3 and Reach are lacking in is the green screen Item, Both Halo 4 and 5 has a green screen but Halo 3 and Reach do not. Now I personally still make Halo 3 Machinimas and a green screen would very useful for someone like me so I think a Green Screen should be added into forge for Halo 3 and Reach.
This is honestly a great idea but not sure it it would be possible since it was not made for Halo 3, it might brake some things if it is forced in for Halo 3.