The issue currently, is that probably only 20% of the playerbase is surviving until the "late game." The other 80% are dying in the "early game" or winter at latest. It is more important to fill the early and mid game with content atm, since that is most people's experience with the game. Early game and Day 1 are also player's first impression so that is a huge priority. The devs are caught in a place where they have to make the late game more full by creating systems that carry from the early game onward, and can be expanded infinitely with a branching progression tree. This way, they can create a lot of content for the casual majority, but also create fulfilling systems and mechanics for hardcore fans. They've laid the groundwork for a lot of these, but haven't made a lot of them as complex as they could be. This is currently the PZ meta as of current IWBUMS:
Day One) Trying to find weapons and food, looking for a bag. Clearing out campsites. Moving furniture. Watching lots of TV.
Early Game) Decent loadout and good gear. Semi-permanent base with relatively good security. Scouting out buildings for better base. One vehicle, probably crap. Reading Beginner Skill Books. Disassembling things. Crafting basic items. Scaving food from urban areas. Preparing for winter.
Mid Game) Most likely decked out survivor survived through the winter with relative ease. Permanent base with good defenses. One good vehicle, and a few part/fixer-upper cars. Might have a generator. Working on chosen skills. Basic farming, foraging, and fishing. Rarely needs to go on food runs. Securing huge area for future base expansion.
Late Game) Best gear in the game. High level combat/agility/fitness/strength skills. Building your own base either completely from the ground up, or walling off cleared out prefab areas. High level security and almost no chance for hordes to get you. Multiple good vehicles, generators, and lots of fuel. Maxed out in chosen skills. Self-sufficient/surplus farm with a few traps and no need to scav food. No real incentive to leave your safe haven or do anything further other than watch the days rack up.
End Game) ???
Imo, the game is really fun, but there is no real purpose to your playthrough other than RP. There isn't a goal other than indefinite security. The interesting thing about this game, and the pull for me, is that the actual goal is death. It's the only game in the world where you are meant to DIE. That's the only way to end the game. The fun for the player comes from trying to die in the most prolonged, epic, badass, awesome way they can. Will you die on Day 2, cowering in your shack with a spoon in your hand as one zombie takes you out? Or will you die in the mall, blasting hundreds of undead with a shotgun knowing you've done everything else you can. That's the appeal to most of us, I think. This doesn't, however, exclude the game from having an end-game. We just need to get creative. The end-game of Project Zomboid should be to essentially confirm the extinction of humanity. To ensure that, no matter what you do in this game, everyone and everything will die. The goal of the game should be to fight against death for just long enough to strengthen the last bastion of humanity, and watch it get wiped out. Maybe you can survive from there, but you would be alone. The last human hoping for just a few more days or years. The current update schedule seems to aim at furthering the game in this direction. Build 42 will bring in animals. Hopefully this means companion dogs, livestock farming, hunting game, and even feral animal enemies. This update alone would add so much variety to the game that it would extend playthrough lifespans by a lot. These features will definitely fill in more of the tailoring, farming, combat, base building, and survivalist aspects too. Mostly, this will just be adding more fun content and laying the groundwork for more advanced AI, without really addressing the late game problem. B43-45 will supposedly bring NPC survivors back with quests, stories, events, groups, and factions. This is the real meat and potatos of this topic. Suddenly, you have stories and events to diversify gameplay. You have your group to help you survive and develop relationships with. You have raiders and feral animals to attack your base. They could have different strategies than zeds so you would need to prepare for all different kinds of defensive situations. Ally and enemy factions could even give you a reason to build different bases. To secure certain resources or buildings, or to have a more strategic position against an enemy or trade partner. If quests are done right, they would be the perfect middle ground between the bigbrain goals you set, and the micromanagement survival stuff you do on a regular basis. Quests could be little things like making 10 spiked bats or collecting 50 logs or finding a 6 pack of root beer for someone. Small things like this would give players a break from the day to day monotony and the intimidating lofty goals they set for each playthrough. This would essentially pose the game as so:
Day One) Trying to find weapons and food, looking for a bag. Clearing out campsites. Moving furniture. Watching lots of TV. Maybe finding an animal or human friend if they're around.
Early Game) Decent loadout and good gear. Semi-permanent base with relatively good security. Scouting out buildings for other survivors. Building up a small, tight-knit group. A decent mount or one crap vehicle. Scaving food from urban areas or hunting and foraging from rural areas. Maybe working on some skills and crafting. Preparing for winter.
Mid Game) Good survivor loadout and gear, decent group loadout and gear. Difficulty finding enough for for everyone through winter. Permanent base with good defenses. One good vehicle and/or mount. Maybe a few part/fixer-upper cars or some pack animals. Might have a generator. Reading beginner books, disassembling things, and basic crafting. Basic farming, foraging, and fishing. Regular food and supply runs to keep up with higher demand from bigger group. Securing huge area for future base expansion. Might have a specialist in group. Defending from zombies and raiders as more people means more noise.
Late Game) Decked out PC. Probably a few decked out NPCs and some lame redshirts too. Several specialists in group (doctor, mechanic, soldier). High level combat/agility/fitness/strength skills. Building your own base either completely from the ground up, or walling off cleared out prefab areas. High level security and almost no chance for hordes to get you. However, strong raiders and enemy factions provide a way larger threat. Multiple good vehicles, generators, and lots of fuel. Multiple good mounts and pack animals. Working on chosen skills. Good food industry from farming/fishing/trapping/hunting, but still need occasional supply runs. NPCs automate a lot of monotonous work. PCs can focus on bigger picture goals and special ops. Bitten NPCs can turn and kill everyone in group if not careful, so no one is ever safe.
End Game) PC has best items in game. Lots of high level, decked out NPCs in group. Network of outposts and bases all under one faction. Multiple vehicles, mounts, and pack animals all supporting trade and travel between settlements. Massive walls and traps and defenses protecting humans from zombies and animals. Opposing factions diminished in strength as to no longer pose a threat. Maxed out in all chosen skills. Massive network of industry and agriculture completely handsoff for players. Self-sufficient society means no need for scavenging. Only threat is a group member getting infected and spreading the disease, but this will not be a restart if there are multiple base locations. No real point to continue on other than to push civilization to its absolute limits.
At this point, we have extended and expanded the game a lot. Although, we still have the issue of no real conclusion point. No moment you can say, "I WON!" My dream "win condition" that fits the lore and themes of Zomboid is this: The Megahorde. The Megahorde will be the ace up the sleeve of the Sadistic AI Director. If you managed to survive all the hordes, surprise bathroom zombies, feral animals, raiders, faction wars, internal infections, the winter, and all the other survival threats; you would be granted one final challenge. This event could not occur until some threshold like population, faction dominance, time limit, or some combination of the 3. Essentially, once you hit that point it would just be up to RNGesus. It might be the next day, it might be a whole year, but it will come. The Megahorde would be a wave spawning horde of zombies that would keep coming until you were dead. Think Last Stand Mode built into Survival. You could probably get yourself to total safety with some cheeky ingenuity, but there's just no way to save everyone. You would have to watch as everything you've built and everyone you know would be destroyed. No point in adding special infected when you can literally have zombies decked out in the awesome gear you gave them. At this point, you would essentially have a New Game+ where veteran players would see how long they can survive in the Megahorde. It might even develop a cool meta of building up your settlements strategically so you can survive longer after the Megahorde. Eventually, the waves will become super intense meaning there would be a hard cap on how long you could push it. This would essentially be the Ender Dragon of Project Zomboid. The place everyone wants to get to, and then start a new game after.