So my suggestions based upon threats, and based on the "Survival" game.
I love love love this game and everything that went into it, love the style of nearly everything the devs have chosen, but I'm a person that stops playing, or plays far less of a game, maybe sporadically only, if the only new things to do are kind of unnecessary.
At this time in Survival mode, if you set up the right base, say you take over 2nd level apartment block in Rosewood, block stairs with TV, put stairs to roof, raise carpentry (for stairs and rain barrel), build farm on roof, put sufficient rain collectors on roof, collect gas cans and generator here and at local gas stations... then its pretty hard to die unless you want to do kind of unnecessary things... which is what happens all the time for me. All you really need to do is go to the gas station once in a while to refill your gas cans for the generator that keeps your water clean and vegetables refrigerated.
So here's my brainfart of stuff, and of course I've read the commonly suggested suggestions:
1. Of course multiplayer, when it is done, on the recent build.
2. Gonna say this just to throw this away since this is on the "no" list in commonly suggested suggestions, but yes zombies with some advanced abilities, or other bad night threats, etc. But, saying this to move onto the next. I can appreciate why you'd stick to your guns on this point, especially if other threats are available.
I saw on the commonly suggested suggestions the sadist movie director "Governor" character NPC. Something along those lines would work. So kind of running on that line of thought:
4. Some flexibility between solo and multiplayer that allows others to log into your game, maybe after a set number of days, as an antagonist and/or help, with appropriate setups, i.e. they load up a little differently i.e. the antagonists would have pre-loaded gear like melee weapons or guns and some amount of provisions, possibly with multiple backstories, i.e. the somewhat sadistic human hunter trophy collector's group that formed that hunts in zombie land for sport, or the government agency that for some reason wants to helicopter people in to kill you to conceal the bad that they did that caused the zombie stuff (or how it is allowed to continue). That helicopter thing may be a reason not to make a completely conspicuous rooftop farming site. Possibly a setting on your solo game (making it a self-hosted multiplayer) to "hostile human survivor mode" leaving it open for people logging in to do bad, and maybe some to log in to do good. Or, servers with events, and factions.
Events are overall good.
5. Just farting out more ideas re: #4, more usefulness of radios and/or ways to detect efforts to detect your base. And of course, the ability to detect a base by bad PC. Bad PC causes a scan of the area, i.e. helicopter flyby, that player, if in the area, can hear And, if detected, a Bad PC or Bad PC team can be flown in to investigate. This may make some electronics stuff more useful e.g. security equipment/motion detector/etc.
6. Could be a storyline related to the above. There could be dozens of storylines giving options for threats.
7. Of course if any of the above can be done via NPCs and AI that'd be super duper, but probably hard to construct. I do see on the common suggestions that NPCs are on the list of yes.
8. I suppose a zombie's ability to break furniture i.e. the holy tv that once placed protects you from all zombies coming upstairs. This could make motion detectors necessary.
9. Or maybe the reverse of bad PC, maybe make farming bad (especially in winter) but friendly airdrops of canned goods are radio'd out. Not sure how that works with the story i.e. does the whole world get zombified? If so maybe there's a charitable group that still has their shit together food production wide.
So absent a stronger, more mobile/climby zombie/supernatural threat these are my thoughts. All I'm saying is with my play style survival has a limit, and some players like me are competition focused and this may be a way that Survival mode can have that.
Love the game though. Great game. And I know all good things take time, and there's a lot of good already in this.