A Galactic Empire need not be officially called that. For example, Asimov's "Foundation" is depicted as having started life as a Foundation of scholars, taking up just one city on one faraway planet and setting up a modest municipal government headed, naturally, by a Mayor. Through a centuries-long series of developments which are the main subject matter of the Foundation Series, the Foundation gains enormous power and territory and comes to rule virtually as many stellar systems as the earlier fallen Empire - but continues to call itself "Foundation" rather than "Empire", and its ruler - though wielding as much power as any Emperor - retains the title of "Mayor".