The developers have the habit of creating in-game buildings and features long before releasing them to the public. On this page you will see pictures of the currently known buildings and features which are already in the game's source files, but have not yet been released to the public.
The following buildings have been added many game versions ago, but have not been released to date:
In the source files of v2.5.15 onwards there are files with 'pvp' (Player vs Player) in their file names. Besides some text files, there are also two images that might give a hint on what this feature is about.