You could explore the game's multiplayer mode if it has one. Or, start a new playthrough with different choices and strategies.
After finishing the Days Gone story, you might consider reading fan theories and discussions online to gain new perspectives. Also, you could look for related content like behind-the-scenes videos or developer interviews.
One option after the Days Gone story is to try and achieve all the in-game achievements or trophies you missed. Another thing could be to create fan art or write fan fiction based on your experience with the game.