Snowyjoe

I guess a good starting point for end game content is to figure out what the "goal" of the game is. A lot of Sandbox games have a goal and the end game is kinda based around those. Like in Rimworld you gotta get off the Rim, in Minecraft you defeat the End Dragon. Just something so you can "wrap up" when you've finished doing everything you want to do and maybe move on. Sticking with the current Lore theme, I guess the first thing that comes to mind is to try and find a cure for the infection or escape Kentucky containment? Maybe the secret government base has an underground facility that contains a key to the cure of the infection that you have to find and get out, but there's hundreds of zombies in close quarters and you need to use everything in your arsenal (bombs, noise makers, guns) to fight through the horde and get the cure and get out of Kentucky safely. If you base the end game around that, then you can start adding more end game weapons, tools, armor based around how players will complete the game. Of course, this endgame goal will be purely optional and just something for players to pursue if they want a challenge in the end.