I love Forge i love map editor and people like me can take this beyond what the game creators could see. we request these ideas.
Pelicans and many other vehicles for useage although not used in Multiplayer matchmaking adding them for roleplay looks and even wars. Phantoms. Shadow. Including while forging you can edit gametypes so you can play as a grunt or jackle or brute even just for the model and it could be a pick up item like a costume in game or could be release benifits only for custom games.
Many people agree you should make us able to spawn trees and a whole lot of them we been craving a actual forest theme and would be fun for servival gametypes.
Forge should also have Oricles able to use a spartan lazor strike that way you can kill people that way for fun 
Additional great things that should be added are Tunnel shaped blocks like ones shown in halo 3 sandbox and ability to control weather like make it rain or snow. if you guys like the best way to do forge mode is to try something like farcry and halo 2 did where any maps made by other players. you download them to your hardrive and this reduces lag of having everyone having to spawn everything rather then having it as its own map leaving the game with endless multiplayer maps then you can do turrain editing and all that fun stuff that will be so far out that 343 will be loved so very much.
also it would be nice to change day to night during gameplay or with a setting change where it changes to night every 30 minutes then day every 30 minutes or however people want. Then you can change snow or add water or even add NPCS.
It seems like a lot of things but this will make halo 4 amazing with these features and like me and all my friends we are dieing to be able to spawn and fly a pelican without a mod. Farcry has a great editing system take a few tips from them
And terrain editor.