Obviously, there isn’t enough grey and black.
Putting aside the sarcasm, I’d be interested in seeing some underwater stuff for the campaign. Like that section in 3s Campaign before Azura, but instead looking through underwater ruins like Jacinto (probably) is now. Or I dunno, an MP map in some sort of underwater research station built by the COG post Locust war(no, you can’t open flood doors to kill everyone). Obviously, that would involve lots of blue, but that’s my favorite color so it’s a win win where I’m concerned. I’m sure TC could come up with some lore background for why you’d be searching through sunken ruins in part of the story. How would they implement any sections where you might not be in a vehicle? Well, uh, look, I don’t work in the “make ideas actually happen” department.
Jungle environments also fit the bill, albeit in a different way.