I think it is very important to avoid making the game tedious, and significantly reducing the amount of resources available on the map is just going to make me move to a new map at an earlier point. It won't make the game more difficult.
The same with reducing the amount of vehicles on the map and increasing the amount of Plague Hearts.
I have a few small tweaks to Nightmare Zone that would make the game more difficult, forcing us to change the way we play the game:
1. Turn off the spawning-in of new vehicles
2. The area has only got hostile enclaves, so to get new ones in, you will have to kill them off first
3. Infestations have 3 Screamers when spawned and those in the vicinity of the base will give you a -10 morale penalty each. I don't think it is necessary to have them spread faster.
4. Plague Hearts in the vicinity of the base also give you a -10 morale penalty each.
5. You don't get influence and resources back when you move to a new map, but you get to take the cars with you, and the resources you have up to the storage limit of the base you leave behind.
This way, we would have to use and take good care of the vehicles that we have because they will be an actual precious resource. We also have to change up the way we usually approach the beginning of the game and what we do after switching maps, since you are really starting fresh and the priority would be to get rid of the nearby Plague Hearts and infestations asap rather than ignoring them like I, at least, do now.