H. Beam Piper's Terro-Human Future History, around the time of Space Viking, after the System States War and subsequent fall of the Terran Federation, but before the first Empire. Scale would probably be different, as instead of small potatoes adventurers I think it would be most fun for characters to be emissaries and traders and Navy commanders from civilized worlds, trying to spread civilization and fend off the Space Vikings. Or be somewhat like Lucas Trask, the Viking with a conscience, trading and civilizing where possible, but still raiding where he feels it necessary for his overall plan.
Jerry Pournelle's CoDominium/Empire future history, either around the breakup of the CD or during the formation of the Second Empire. Or during the Secession Wars. Lots of interesting periods to play in, just avoiding the ones where everything is too stable and boring.
Those are the settings I would most like to GM or play in; how about you? What setting would you like your next campaign to be in?