Hopeless Frank

I'm all for making the game harder as long as you are doing it by using:
 
1. Realistic threats (with realistic counters).  Food shortage?  Sure thing, as long as food does not rot or vanish artificially quickly.  Disease/Illness?  Sure, as long as you get sick realistically, and/or there are in game countermeasures where they would be appropriate.  There should not be ANY unrealistic threats except for the zombies themselves - otherwise this stops being a game about the ZA and instead becomes something else.
 
2. Tougher zombies.  This doesn't have to mean making the zombies less canon - the ways you've discussed here sound great (improving the AI for gathering/creating hordes, new hordes migrating in, etc.)
 
I totally get that the game has become easier because the amount of survival techniques have improved without ramping up the overall difficulty.  I just get worried when people start talking about 'it should get harder as the game goes on' because it makes me think we're talking about some meta-game difficulty slider that just cranks things up as you go on, rather than by producing a realistic survival scenario where things MIGHT get harder as you go on, but where that isn't a necessary outcome because the game is simply coded that way.  The game I have ALWAYS been waiting for is the one that presents the zombie apocalypse in the most realistic world possible, and I hope PZ doesn't end up moving away from that to make all the 'this is too easy' people happier.

I think random moving hordes, like the hordes in TWD, would be a constant up in difficulty. But please make them not come to me no matter what I do... if I choose the (very boring) playstyle far-out-in-the-woods, I should be relativly safe. A horde wandering out far in the forest should be very uncommon (maybe they followed a helicopter flying over them, and had no other stimulus since then so they still wander in the direction the heli flew-stumbling over your unlucky toons camp).
But if I stay in urban areas, wandering groups of zombies should be a constant danger, as should making noise be.
 
Lategame new accomplishments and dangers could go hand in hand, making the game having more to achieve but also new drawbacks and dangers.
 
Like, if I build my base, start farming and start to sustain myself that way, I should also have new goals like getting a generator running to use the fridge again. Or getting a car running and pimping it mad max style.
For the generator for example I would have to go out and scavange again, need a car (if introduced) to get the generator from the hospital/shack in the woods/mall/ etc etc to my safebase, need fuel and spare parts. The search for parts and stuff would be dangerous if the hordes are back, so a new way to die even if late in the game. Also the generator makes noise and whatever code determines how hordes move could take the generators noise into account, upping the chance a horde moves to the safehouse.
 
I think lategame, the safebase playstyle should still be viable, with punji stick traps, walls (like brick walls) no zombi can destroy and other ways to have it really safe. To get the player killed, I think it's much more rewarding to have the player choose if he wants to risk something for something and die in the process, and not to make death absolutly inevitable in cheap ways like a constant increase in zombie population or needed items just vanishing from the map or breaking without replacement.
 
Also, later in game, the wildlife creeping back into town could create new enemys added to the zombies. Bears or wildcats could be a rare but dangerous encounter for the lategame.
 
Another way (if hordemovement is back) could be a real big horde moving from one edge of the map to the other. This could simulate giant hordes wandering across the continent. This could happen ramdomly after some time has passed. Like, after 3 Months (the time needed for these megahordes to form) the new danger "megahorde" is introduced like now the watershutdown. A megahorde would be like a natural desaster, to be avoided at all costs or fought to the death.
 
But, what I disliked about the hordes that tore down my base everytime, was that I had no way to make a zombi barrier at all, and that should be possible. I don't like that zombis can break through everything I've build. To build a base that no zombi can tear down and starve inside it besieged by 400+ Z has its own horror. I would have to fight my way out, loose the horde and travel to a new town... and someday I might come back to my old safehouse with the horde gone.
 
Nearly every Zombimovie has the survivors building their base besieged by Z. I want to have this, too. Of course it's not my game and you guys can do what you want with it, but I think the "Would I survive, and how would I do it?" is a strong part of the fun playing a zombie game, and if I knew the game would kill me later, no matter what I do, it would kill the fun (for me at least).

I think gaunti12 hit the nail on the head with his suggestions.  I would love to see wild animals, megahordes, and the like in the game as well.  But not neglecting NPCs, I think that they will also provide an interesting late game dynamic.  As the NPCs get more desparate, they should be more aggressive, and they might make the late game much harder (provided they survive, that is).  
 
Also, with the erosion mod being implemented into the game, I think making farming not an option during the winters would definitely make it more challenging as you must survive for roughly three and a half months without any extra food source than your stockpiles.  Imagine a megahorde attacking at the beginning of winter.  You are basically stuck in your base forced to either brave the horde or dwindle away your remaining supplies, hoping that something else draws the horde away.

and is said that russia did so where exactly?
my point was simply, that the propaganda we see in our evening news about this conflict is - this time - even less believable than the russian one...

As a fellow european internet-expert i could only say, listen to that guy above me. If u think he is wrong watch a season or two of "House of Cards" and think again... Neither side of this, Nato or Russia, is telling the truth  or has "honest intentions" but one has to admit that this time the russians are far more reasonable in arguing and also have far less direct lies in their propaganda than the gone wild Ashtons, Rassmussens or Kerrys from "our" western / nato side of view.