This is something I have thought about for a long time. It depends on something likely impossible.
Forge is a gamemode from Halo 3. It is basically GMods Sandbox, but with some minor differences. Forge’s main beneficial difference over GMod’s Sandbox is the editing, and relocation, of spawning props, and player_spawns.
Ideally, Forge would directly create a .BSP from data changed INGAME. I don’t know if it is possible to edit, or create, a BSP inside of the Source engine.
These are the two real questions:
Is it possible to change or create spawnpoints ingame using Lua or any other method?
Is it possible to edit or create a BSP from within the Source engine?
Forge mode is again, relatively the same to Sandbox aside from the spawns.(UI excluded)