Well I havn’t seen a real constructive thread about this yet but please add a forge feature you would love to see in Halo 4!
Here’s one that I’ve read back on the Bungie forums back before Reached shipped.
Please add a despawn timer! It would be a great addition for Forge.
So go ahead, what would you guys like to see? (Be realistic, no pelicans, scarabs, anything very massive, or unlimited items/budget.)
An undo change button.
Add a grouping items option to make it easier to move large structures around and make it in such a way that you can make custom vehicles.
I disagree; I think the Pelican is totally realistic. The Covenant Wraith, though, not so much, unless they redesign it with multiplayer in mind (which they’re already redeisigning weapons and vehicles anyway). Well, even if they aren’t added, at least Frankie promised us at least one new vehicle that would dwarf the new mech in War House.
-Grouping items to move them all at once, plus the ability to mirror one side of an entire map in both inverse and standard symmetrical modes.
Reason: I got sick of spending countless hours designing and building symmetrical maps and then after having built everything to decimal point precision on one side, having to calculate the decimal points all over again and carefully mirror every single object on the other side of the map. I seriously lost sleep over this. 
-Undo button
Reason: Self-explanatory
More than anything I really do hope the 343i cut down the number of inherent polygons in each object. Bungie really went overboard with this and it was a detail which people really did not need. Moreover, they just led to massive framerate lag problems, even when used sensibly. The large number of inherent polygons in some of the single objects just made them impossible to ever use, without causing huge framerate drops.
Also, some of the objects (for instance 3x3 Blocks) actually contained inherent framerate issues, without even using anything else. I really hope 343i have taken note of this and completely addressed this sort of thing at a fundamental level.