It has been a privilege to reread and post about Poul Anderson's World WithoutStars. However, those premises require much more than just a single short novel. An ideal Poul Anderson multiverse would contain not only an indefinitely extended Time Patrol series and Technic History but also several long series spinning off from (at least) WorldWithout Stars, After Doomsday and the Old Phoenix and the pre-history of Ys.
But, in order to write all that, Poul Anderson would require the antithanatic. And, with an indefinitely extended lifespan, a man like Anderson would probably, after several centuries, turn his attention away from writing to other pursuits including, if meanwhile an equivalent of the hyperdrive or space jump had been developed, exploration of the real universe.