Surviving OVERKILL's The Walking Dead involves a lot more than completing missions and leveling up the playable characters (and that extra secret one!). The layers of progression in this four player co-op shooter also involve upgrading the camp, the home of the protagonists, allies, and rescued survivors.
To succeed, players must also acquire camp upgrades for The Hub, Depot, The Range, Radio, and Clinic. The upgrades open up the full map and access to the game's story missions, and provided upgrades for the individual characters associated with each Work Station.
Where player skills are upgraded simply through spending skill points earned from leveling up, camp upgrades require various resources (mainly scrap) acquired from playing and replaying missions and expeditions. The tricky part however, is maxing out the camp upgrades since there's an additional cast: blueprints.
Blueprints are considered currency, and the game says as much in its interface, but they're super rare and hard to come by. The only way to get them is by talking to Anderson at base camp (the same person who players must slowly visit in between each story mission to claim prizes and unlock the next one) and checking out his bounties. The system is problematic but is a necessary evil based on the game's design.
Add bounties every time you see them and complete them. You must load up all five and complete all five in order to redeem them with Anderson and when you do, you get blueprints to spend on those top tier upgrades.
And once the highest level camp upgrades are unlocked for each Work Station, the skill trees for each character can then be unlocked as well.
Note: By acquiring more and more camp upgrades, the daily upkeep cost for the camp also increases. That means you'll be required to keep doing missions to stockpile resources and keep moral high.