More Forge!!!
Terrain building.
Ability to add weather Effects.
More advanced lighting.
No crazy limit on Grav Lifts! If I don't use all the vehicles, free up space for other items I do use.
Copy and Paste for objects.
Being able to align objects and then lock objects so I can move things in bulk.
Way more objects. If its in Halo 4, it should be in Forge to play with.
Multiple different skins for objects. More than just gray.
Forerunner, Covenant and Human architecture.
Sort out the framerate issue when too many objects are linked in one place.
More options for gameplay customisation.
Make adding/building gamemodes way more user friendly.
While I did love Forgeworld, just give us a variety of different size and shaped worlds to choose from. In the end, people did not care about all the background stuff.
Ability to add Sounds to levels.
Have some prefab events built into the editor. Exploding walls, breaking doors, collapsing bridges etc. Would make building Invasion maps way more interesting.
Adding A.I into the map for FireFight maps or being able to add A.I ships that fly over various maps.