Ok, I know some (maybe most) of these are already posted, but I'm just going to list my opinion of new forge features.
Edit game type in-game- This will allow the game type to be set up EXACTLY how players want it to be, since they can check how the settings affect the players movement throughout the map
Use of terrain- This will allow players to TRULY make a map. A true custom map is where you set the terrain, and all the items in it. Use of water would make this perfect.
Weather- What, can't Spartans aren't allowed to play in the rain?
Night time maps- Neither the makes of Call of Duty, nor the makes of Halo have figured this out yet. How about a night time map? Wouldn't it be sooo epic if you could use night vision to help you see in a very dim map? And it would FINALLY put use to the lights on vehicles.
AI- Who doesn't love killing Grunts just because it's fun?
Blood painter and corpses- Imagine this: You're playing a custom game of Flood (Infected). You're slowly walking down a hallway, when out of the corner of your eye, you see a trail of blood. You follow it carefully, and as you turn the corner, you find the body of a marine. You investigate it for a second, and then... WHAM!, a flood kills you. Scary... (Thought red fit this one better )
Bringing back old vehicles- Aaaah yes... the good ol' Covie motorcycle... the Chopper. Or the fun to use Elephant? Also, what about the Covenant troop transfer vehicle: The Shadow, and the Spectre? Halo 2 times, baby!
Dynamic parts of the map- Wouldn't it be cool if parts of the map moved? Whether it's something swinging around, or something flying by the map, it'd be pretty cool to see.
All items in all maps- This is one of those things that just makes sense. Let us be able to forge on ANY map we want, not just the maps with open space.
More items- I'm so sick of the same objects being used over and over again, and it only being those items. Let's see some new items, like stuff that looks like it's from the Covenant.
Combining objects- Yeah, Halo 4's Duplicate Object is a great attempt to help forge, but that's all it really is: an attempt. It doesn't help people when you can ONLY copy one object instead of multiple items.
Larger maps with no limit on objects- People need room to be able to forge masterpieces. We also need to be able to use some objects MANY times. 100 is the limit of some of the groups of items... making an epic base needs more than that.
