"He thought back to the days when he had ridden ships through yonder spaces, bargaining in strange cities or stranger wildernesses, or beneath unblue skies and in poisonous winds, for treasures Earth had not yet imagined. For a moment, wistfulness tugged at him. A long time now since he had been any further than the Moon..."
- Poul Anderson,The Van Rijn Method (New York, 2009), p. 147.
This is in the story that introduces van Rijn. So here is a potential series: how van Rijn, born poor, worked as a spacehand and became rich. Anderson's History of Technic Civilization contains several proto-series, single stories that could each have initiated a series, and potential series, chronological gaps that could each have been filled with an entire sequence of adventures. Van Rijn's earlier life is yet another in the latter category.
Hercule Poirot retired as a uniformed policeman before becoming a private detective and I have heard that Agatha Christie realized that she had a problem, writing a long series about a character who was old to begin with. Anderson shows us the older van Rijn and the younger Falkayn as they age. They remain active and learn as the world changes around them.