Escalation of World Threat
Do you remember your first night in Minecraft? I feel like everyone does. Preparing before it gets dark, and the beautiful procedurally-generated wilderness that surrounds you starts to get filled with hostile zombies, skeletons and the infamous creeper, all the while you're not even certain you'll find food for the next day. There's nothing like that first night in Minecraft for a sense of fear and desperation, including all subsequent nights in Minecraft.
The survival game genre suffers from a widespread flaw: once you've overcome your initial situation and figured out how to survive, there's not much point in continuing to play any more. The most successful ones (like Minecraft) usually transition into some other kind of game, more about building, or adventure, or community play.
Project Zomboid (PZ) actually bucks this trend, for the most part. The first time you play, you probably get infected or killed on the first day, by some of the first zombies you encounter.
But, after that first day, you learn how to scavenge effectively you learn how to fight zombies, you learn how to avoid them and find and barricade safe shelter. You start to get comfortable, just like in other survival games. Then you experience the Helicopter Event.
The Helicopter is just one of many ways PZ throws a wrench into your efforts to survive. These methods can be described as a category as Escalating World Threat, and they come in two forms:
Taking away nice things
At the start of the game, every house has electric power. The TV's are working, as are the fridges which are usually stocked with food. For the first few days you can learn skills watching TV, scavenge residential and commercial kitchens for fresh food, and every tap has an infinite supply of clean, drinkable water. Gradually, as you survive for longer, the game starts to take away these niceties, forcing you to think more carefully about what food you choose to eat now or store for later, to think twice about using that clean drinking water to wash your bloodied gloves, etc. You are compelled to find alternative solutions for the problems that these things used to solve for you.
Increasing dangerous things
With default settings at least, the first day of Zomboid is at the lowest zombie population. The population will however gradually increase until it hits a peak (28 days after start by default), so that you see ever larger and larger hordes. The respawn settings will also eventually repopulate to this peak level so that you can never completely clear the area in perpetuity.
Late Game: The End of Escalation
However, after about a month in-game, PZ somewhat falls back into the same pitfalls that other survival games do. By then, you've managed to survive the helicopter, the power and water is off, the fresh food mostly rotten and the zombie population is at peak. If you're surviving now, you can probably survive indefinitely. Yes, winter is still to come, but it's several months out and unless you're just barely getting by, it probably won't hinder you much.
This is the real problem with late game PZ, the end of escalation. As far as I'm aware, the upcoming updates do not address this flaw. They add more in the way of immersive survivalism, but not in the way of making it harder to continue surviving the longer you already have. Even the long run plans don't seem to address this: NPC survivors might add more things to do, but probably will make it overall easier to survive rather than harder.
To fix the late game lull in PZ, the devs would need to focus on extending the escalation of world threat that the game already does so well in the first few days/weeks. It could come in the form of altering the zombie lore the longer it goes on, making some of them stronger, faster etc. Or those planned NPCs should be increasingly hostile/dangerous the longer the game goes on (eg. military faction?), perhaps sometimes even forcing you to abandon your base if the region of the map becomes too dangerous.