Uniqueness

My two favorite kinds of science fiction are time travel and future history series: Wells and Heinlein.

Time Travel
There are two fundamental premises: the past either can or cannot be changed. Poul Anderson systematically examines both. In The Corridors Of Time and There Will Be Time, rival groups wage war throughout an immutable timeline by changing the significance of known events whereas, in the Time Patrol series, an organization prevents change in a mutable timeline.

Future History Series
Robert Heinlein wrote one Future History, although five early Scribner Juveniles share a background with each other and with the "Green Hills of Earth" period of the Future History. Thus, these five novels might count as a "Juvenile Future History."

James Blish wrote a four novel future history, Cities In Flight, a four story future history, The Seedling Stars, and the non-linear Haertel Scholium containing three distinct future historical sequences.

Larry Niven has the Known Space and the State/Smoke Ring histories.

I think that other future historians have one series each except Anderson who has at least eight. I am here counting the single novel, Genesis, as a series because its successive chapters cover geological ages. Anderson moves away from histories with FTL, aliens and interstellar empires towards histories with STL, no aliens and human/AI interactions.

These thorough treatments of time travel and future histories make Anderson unique among sf writers.

Designing A Universe

People used to imagine a breathable atmosphere not only on other planets but also between planets. In Karen Anderson's "The Piebald Hippogriff," a boy rides a hippogriff through a blue sky to a cultivable Moon but this is unadulterated fantasy. David D Levine's contribution to Old Mars is sf set in an alternative universe with different laws of physics. Lifted by balloons, Captain Kidd's ship sails through the Terrestrial, interplanetary and Martian atmospheres down to the surface of Mars where the inhabitants, for a change, are not humanoid but crab-like. (We know that we will get Martians but not what they will be like.)

We are not unfamiliar with interplanetary atmospheres. A breathable atmosphere stretches between planets with a common orbit in an ERB work and Larry Niven's Smoke Ring is a torus of breathable gasses encircling a star at a planetary distance. However, Levine's Solar atmosphere requires different laws of gravity. Thus, this story about an alternative Mars also presents an alternative history and cosmology. The cosmology could be the premise of another themed anthology.

If Poul Anderson had contributed to such an anthology, then I would expect his story to outline the alternative laws of physics enabling the Solar gravitational field to hold a breathable atmosphere enclosing the particular planetary atmospheres. Anderson might also have presented a natural philosopher speculating about an alternative universe where the planets were instead separated by the Lucretian void. Anderson's vast corpus of works enables us to speculate about how he might have contributed to later trends in sf.