I have gone through these ideas with my friends over Xbox live they agree that these would be awesome in halo 4 forge.
1.) remove the budget idea and replace it with memory for exampled it would say how many MB aloud to be used on that map and it would count down to 0 when it reaches 0 you cannot place anything on that map.
2.) to have the ability to change the texture of a map so instead of having just a snow world you could change it to sand grass snow ect. or just put them all in 1 map.
3.) to be able to forge bigger vehicles like the scarab.
4.) like in halo reach you can change part of an objects colour but in halo 4 you should be able to change the material an object is made out of.
5.) change of weather on a map or/and day and night cycle.
6.) the ability to create the size of a block.
7.) like in halo 3 you can make it so only the leader can forge well i think in halo 4 you should be able to pick who is aloud to forge in your game.
8.) the ability to change the game type in forge like in halo 3.
9.)more options before you start up forge.
10.)space forge e.g. were you can build stuff in space.
I hope my ideas help you with some part of halo forge after all forge is what makes halo fun for years.