This is a list of Unreleased Buildings which may be seen from Kixeye's forums or their database:
Unreleased Inferno Buildings
This is a list of the unreleased Inferno Buildings. Currently, there is 1 unreleased building found.
Unknown Inferno Building
This unknown Inferno building is only seen when you do not have an Inferno yard yet and you are viewing Moloch's bases in the Inferno Cavern. But when you attack one of Moloch's bases, it will instead be an Under Hall instead. Some players have speculated that this building is a possible Infernal Outpost. KIXEYE claims that they are working on a World Map for Inferno. It is unknown wether this building will be used for the map or not, and how the upgraded map would be accessed.
Unreleased Overworld Buildings
This is a list of Unreleased Overworld Buildings. Currently, there are 2 unreleased buildings found.
This kind of Fortification was scrapped for unknown reason.
The Fortification, however, is supposedly used for the Housing Building, because the Fortification size matches the size of the Housing.
It is unknown whether this type of Fortification will be released in the future or not.
This unknown building is shown in a preview when you click fortify to a building. It is most likely that this represents a random building, and not a new building for the future.