The recent Mass Transit update has paved the way for basic air travel in Cities: Skylines with the introduction of blimps. Taking this as inspiration, modder Armesto has created a series of mods that use a custom blimp model to create Zeppelins.
As well as a stationary Zeppelin that can be used as a building or an attraction, Armesto has created a Zeppelin Depot. It has been based on the old Friedrichshafen hangars in Germany.
You must have the "Mass Transit" DLC for this mod to work.