There are a lot of things I really want to see in four's forge mode..
-Triggered mechanic items. Like doors, drop pods or light bridges
-A properties setting that keeps displaced objects from despawning (crates wouldn't despawn, and building blocks wouldn't freeze once at rest, or despawn, or both).
-The ability to give certain objects motorized properties- Imagine how creative forgers could be, making a piece rotate like Ascension's antenna, or maybe a conveyor belt. If you were really smart about it, you could build a pelican, spawn points and weapons inside, and have it travel around a set path in the air. Or build an out of control train like terminal.
-I really enjoyed that one about grav-lift trajectory. Maybe add a grav cushion to the other end to make high altitude grav jumps non-lethal?
-More aesthetic object choice, like bodies and bloodstains and destroyed vehicles. Lots of trees.
-Preset background objects, so we don't have to build giant buildings to backdrop our city maps. Maybe something like those half finished backdrop buildings in Outskirts on Halo 2 that you could explore for hours around.
-Different texture settings for wall/floor/surface objects so our maps don't look so identical.
-Landscapes as items. Add a grid sized block of tundra or desert or grassland.
-I can see this being a bit of a stretch, but a more realistic physics engine for water, so you could pool or redirect it. Wouldn't that have been so cool on forge worlds waterfall?
-Add water to areas maybe?
-I highly agree with the 'select all' idea to fuse pieces together.
-Scalable/editable objects. Size a coliseum wall down to half, or cut off a chunk or even put a hole in it.
-Introduce large vehicle gameplay- Scarab/Pelican, bring back the elephant. Not the spirit or the scarab though, they're too big.
-Signs that scroll text! Windows marquee screensaver meets Halo. It was sort of in ODST's campaign I think.
-One huge one for me, have a setting to forge maps with no budget/ object limit, but these unlimited maps will not be chosen for matchmaking, you can only play them online in a customs lobby.
-Buyable (but free works too) dlc forge item packs- you can play on maps containing these items if you don't have the dlc pack but you won't be allowed to edit the map unless you buy the dlc.
-More breakable objects (damn we've been missing those pallets)
-Undo button. (*delete on accident* "CRAP")
-Lock objects so you can't accidentally move them. Halo 3 floating objects, I'm looking at you.
-Despawn control/ more efficient respawn control for items
That's all I can think of for now, but I definitely have more to think of and add later.