They'll have a great deal of power to create environments and NPCs – they can even control any NPC directly to speak with players or provide a more challenging combat encounter. This will be a separate version of the game that lets any player with the right admin access act as a game master on a given server. These might be fixtures on specific servers, downloads, or instances that players can jump to via the in-game starmap. Just as the structure of Starbound as a whole is inspired by console platformers like Metroid and Castlevania, so will players be able to create and distribute their own Metroidvania-style games. Using simple logic devices, players will be able to create monster spawners, signposts, and so on.
On the list are player-made missions: dungeons or quests that can be built within the game itself then pulled out and distributed as standalone challenges.
Modding has a role to play in making that a reality, but in-game tools are also a priority.