here. "A Style In Treason" by James Blish and the early pulp magazine Dominic Flandry stories by Poul Anderson are good examples of works that are enjoyable to read but not presented as plausible futures.
Blish's High Earth-dominated confederation served - or betrayed? - by a thousand year old "Traitors' Guild" confronts an alien interstellar empire called the Green Exarchy which:
"...drew tithes from six fallen empires older than man..."
-James Blish, Anywhen (New York, 1970), p. 19.
Anderson's Terran Empire, served by the heroic Dominic Flandry, confronts the green Merseians with their interstellar Empire and plans for galactic conquest.
In these works at least, neither Blish nor Anderson seriously proposed empires as probable forms of interstellar government. They were instead exotic settings for futuristic adventures. "Earth" is not enough. It must become High Earth or Terra. The language is colorful and, coincidentally, both sets of villains are green. Or, rather, the Merseians, like ERB's Tharks and Dan Dare's Treens, are green whereas the mysterious Exarch is "Green."
"A Style In Treason," in particular, reads more like an alternative universe fantasy than like hard sf. However, despite Flandry's pulp origins, Anderson was able to transform the Flandry series into a serious dramatization of the reasons for the rise and decline of civilizations. The series matures with its hero.