Doctor Who. Although involving interstellar space travel but no time travel, Jerry Pournelle's War World shared universe is obviously designed for a similar purpose. These are extracts from the blurb on the back cover of the 1988 Baen Books edition:
"A WORLD FOREVER AT WAR
"Born of rebellion and civil war, Haven is home to diverse races.
"*The Saurons. A race of militaristic supermen...
"*Apaches who are forced into an alliance with sea-faring descendants of the Vikings in a desperate attempt to fend off the Saurons...
"* The last regiment of Imperial Marines."
Of course Poul Anderson is able to contribute to this series and of course his contribution is not what we expect. It is he who writes about the Marines yet his story contains no combat. Instead, it addresses divided loyalties and the effort to get the Marines back into space so that they can defend the Empire or at least what is left of civilization.