One thing I think is very important for SoD2 is that new features aren't making it tedious to build up a community. Further reducing the rate of influence gain, standing improvement, skill improvement, cost of building facilities and upgrade them, item crafting and all that, isn't going to make the game more difficult per se. Just slower.
A small reduction is perhaps in order, but one single addition to Nightmare is all that is needed here; get rid of the 1-minute zombie sieges.
When in the yellow (mid-tier threat level), you get 2-minute sieges which include 1 Juggernaut, at least 1 full-size horde, at least two waves that has a chance of including either a Feral, a Screamer/Bloater trio or a small plague zombie/armored zombie horde in addition to a bunch of single regular zombies from all sides.
When in the red zone (but not maxed out), you get a 3-minute siege with 2 Juggernauts, at least 2 full-size hordes, at least three waves.
When threat level is maxed out, you (you have a chance to)get a 3-minute siege with 2 Juggernauts, a Feral trio, at least 3 full-size hordes and at least three waves that does include a Screamer trio and a Bloater trio each.