You can already change settings so much in firefight like infinite health, rockets, bottomless clip, spawn only hunters for maximum points etc.
To solve spawning issues all they would need to do is divide the base map into a few quadrants and make it so you to have to place a landing/spawn spot for covenant (or flood (YES PLEASE!)) drop-ships or pods to spawn enemies in those quadrants during each wave, round or whatever.
OR they could have preset locations on the map where a forged item can't be placed because it blocks a spawn/drop spot.
Problem solved.