Well I know a lot of people don't see the point in this, but I'd like AI/NPC units in Forge mode. To clarify, I'd use it to setup maps where a group of units would endlessly respawn and try to overtake a stronghold from AI allied with the player.
Maybe it's just petty amusement, but watching a group of well-equipped Covenant soldiers hold out against a Flood onslaught is entertaining enough for me to spend hours playing that map, if it were designed well enough. Y'know, with barricades, layers of defenses, a complex defense structure in general.