survival mode (with saves and an option to enable or disable permanent death), you can play with your friends or with a squad o bots, and everyone has a certain role in the game, and the whole objective of the game mode is to explore levels and survive, and when all the objectives, for example: complete the group X quests in level 1, collect resources, set a camp in level 2. And when are done you complete the game mode, or you still can play it. like when you beat it "the forest" but you can still play, like in this game mode you could actually build outpost and find random outpost bases and abandoned colony's, with npcs, having a basic aggressive or friendly behaviour, like you can be friend of a group X but because of that you are enemy of group Y. In the game mode you could have ways to go back in other levels, and have other type of enemy´s not only bacteria but bandits in level 1 or armed face lings
and about the class system, in the start of the game every player could have a chance of choose a certain class, and when playing offline the player can assign in the start of the game what classes the bots will have. And all classes have certain equipment's that others don't have.