I’m pretty sure that the customs community has pretty much made the point clear:
Forge in Halo 4 was a bust 
So what would everyone here want in Forge?
Mechanics and Option
There is an option for dynamic lighting to take place, and if not toggled, will not be used in custom games
The Forge mode basics should all be retained from Halo Reach:
-Fine Editing
-Degrees
-Physics
-Colour
-Zoom
-Object Lists
-Boost
and from Halo 4…
-Magnets
-Lighting
Improvements or Additions
-Less Sudden Movements causing giant jerks of the camera
-Item Grouping
Selection of different various objects and turning them into one big object.
-Custom textures
Different or custom textures for any part of the canvas on PC via cloud or the gametype anvil(see Terrain Editor/Dev Kit)
-Custom Objects
Saving any grouped items(props like in Gmod)with their budget scheme.
-Filters
Light Filters(Reach)
-Lights
-Terrain Editor/Dev Kit
Could possibly be done on PC via cloud or an in-game gametype of forge, anvil.
-Cutscene Editor
(see above post)
-AI
Placeable and programmable AIs with weapons or traits of your choosing.
-Online Custom Games
Matchmaking where you can choose which lobby hosted by ___ without invite.
Host can boot any unwanted players
-View Changer
Ability to choose between 1st and 3rd person…and eagles’ eye.
-Picture for map
Actual ability to have the map preview picture available
-Community Map/Gametype Spotlight
Self explanatory
-Custom Sounds
Ability to add custom sounds
-Custom Gametypes
-Additions of Space Battles, Race, Territories, Invasion, and Headhunter.
Technological Advancements
Due to the new power of the Xbox One the following will be available
More and placeable AI
Bigger budget
More Objects
All of the improvements listed above.
What do you want in Halo 5’s Forge
…so things like this don’t need to happen(it is very constructive if you look at it in some ways…)