Creating a modding API to make for example: guns, modules, texture packs, enemies, maps, and etc. Then add the ability to upload the mod to your private server or in the steam workshop with a mod-manager in the main menu.
Steam workshop is planned i assume (pure guess) that they will start looking into this after coop is out. so maybe one or two updates after coop? Id love to have a modding community though!
Pretty much what Hab said^^
Steam Workshop and a Modding API are most definately planned now, Can’t wait to see what some of yall make!