What will add the most to forge by far are the first and last suggetsions. Less restrictions on objects, and a large, flat terrain surface to work on.
What I was really hoping for from halo 4 forge was the ability to make detailed BTB and Invasion maps completely based on forge objects. It seems however that this is even less possible now than it was with Reach.
A lot of people don't seem to understand how fun big maps can be. Especially with Invasion where you can have multiple large areas that are all very well utilized as you progress through the phases. And when I make a big map, I want it to be completely orginal. I don't want half of it to be something everyone has already seen hundreds of times. I'm not saying we need anything near the size of Forge World in Reach, we just need a big empty space.
The amount of detail you can add right now is extremely limited. Reach's object limit made detail very restrictive, and Halo 4's looks exactly the same. Adding lots of small detail is what makes your map feel unique and artistic. With most categories not even reaching 100 pieces each, the amount of detail you can add in a large area is just pathetic. Especially if you are already using forge objects for your base. I am happy with the variety of pieces that are provided, we just need a lot more of the same thing.
These two suggestions are by far the most important areas to improve on and are the defining points on how much we can do with forge. Most other suggestions I see are just way too hard to incorporate, or very meager compared to this.