Scripted respawning
Always wanted to see a single use respawn that uses neat effects, such as ODST pod dropping in or a pelican landing zone that drops players in a wave.
AI placement/scripting
Simple placement with respawn timer options, etc or more advanced placements such as AI scripting that you can link to set patrol routes or move points.
ALL OBJECTS/VEHICLES/ETC
It’s painful to see so many objects in the game that are not usable in Forge. Same goes with vehicles.
Terrain editing
Placing terrain objects is great, but would go best hand in hand with even the simplest terrain tool. Also, water.
Emitters
Scripted entity that emits more than just pretty effects. Could emit damage dealing stuff too, such as shooting a rocket, wraith mortar, needlers, grenades, etc. Set to shoot in a particular direction, angle, or even track players/etc like a turret. Make some cool out-of-bounds stuff like the plasma turrets of snowbound!
Other than that, I love Halo 5’s forge and since the same folks who are responsible for that are handling Infinite’s forge… I am sure I will be pleased.