Poul Anderson was a contemporary of Asimov. Anderson had a whole 'future history' about the Terran Empire.

Also have a look at Adrian Tchaikovsky's dogs of war and Children of time series.

Its really scratching my Asimov itch of late.

Larry Niven developed the "Known Space" universe, with many planets having interesting societies based on their environments. It's neat. Not really a central theme of the books, but it's in the background.