I just play 7D2D singleplayer and am not normally very active in the community aside from browsing dev posts - which are in pretty short supply lately, so if this is something that is already discussed, forgive me for bringing it up again.
I find that once I have acquired a gyrocopter, I have already gotten the best or close to it of everything else and lack any incentive to actually use my shiny new vehicle to fly around as there is no longer anything for me to strive for.
It seems from very brief searching that getting the gyrocopter is frequently an end goal for other people too and the game is effectively over once attained.
I don't want to end there when I finally get to fly, I want it to be the beginning of my greatest adventures.
So my point is that there should be some sort of additional goal that exists even after getting all of the best gear. A MacGuffin perhaps, that has no purpose other than I want to get it. I already like to put a safe in my base and stash old cash, silver, gold, and diamonds inside it, but while this is a small bit of fun, there is no real goal here aside from "more is better."
I could go for max level, which apparently is 300, but if the formula for leveling provided by the wiki is accurate, that would require 25 billion experience. Not a realistic goal.
One idea that I had is a zombie kill trophy list. Every type of zombie in the game could have a counter for how many of that zombie you killed. For example, you kill 50, you get a bronze trophy for that type of zombie, 100 for silver, 200 for gold. Sure, killing 200 departed women might not take too much effort, but getting 200 demolishers will take a bit more time (the numbers could be set higher if feral/radiated versions counted for more than 1, but the goals should not be so high that they aren't worth actively pursuing).
I suppose you could get a small reward for trophies, like a +5% damage toward that type of zombie for each trophy (up to 15% at gold), but that would not really even be necessary, as they would just be an arbitrary goal to strive towards when everything else is already done.