Im a massive fan of Firefight, in facts in the main thing I play on Halo Reach.
I hope it returns for Halo 4, also hope the flood are added.
Gears 3 has a 30 second gap between waves where you can set up barriers, laser fences, mines etc.
That could be a good thing to add to firefight
also add Firefight versus (server based)
The firefight playlist for me is near perfect, so it can only get better