This is assuming there will be few differences/updates between Halo 4 and the next iteration:

Ability to select any item, structure or weapon for instance, press a button and have all similar items light up with a waypoint or lit up wire frame so you can find missing pieces. For instance, it would be very useful for finding weapons and spawns within geometry. You could highlight all FFA spawns, or Blue Team Oddball labeled spawns when revising a map's spawn layout.

Grids: 8 Grids total with the ability to select from several different sizes, somewhat like how Energy Doors are provided. 40x40, 5x40, 2x10 . . . down to 2x2 for windows. Grid becomes either invisible, or with a very very slight energy wave animation, but WITHOUT THE GRIDLINES in game mode. They should not obstruct your view when standing close to them. If you are familiar with my live broadcast maps this will make more sense. http://minus.com/i/Ns6I8KwIOq4t

Wall items which can actually be used as walls and platforms. Like Brace Large without the angled piece (or the bumps). Size should be 1.4x20 with minimal decoration

I'm sure this has been mentioned in every wish list, but we must have the ability to select and group items, like entire bases including their objectives and spawns, and move them as a single unit. Being able to duplicate (copy/paste) grouped items would be a nice added bonus.