Since you asked for suggestion, I think you should add like factions, some ally and some are enemies (like survivors, The Lost, Army/Police ect..).
The black out should only affect building not cars, I remember a mod that did this long time ago.
Add hard mode, something like faster zombie/Animals.
Herds?
This could be impossible but disabling traffic and instead make them spawn randomly in various degree of damage.

Other than that it's a pretty awesome mod good job on it!