Full Idea
One can be a realist about possible worlds without adopting Lewis's extreme views; they might be abstract or mathematical entities; they might be sets of propositions or maximal uninstantiated properties; they might be like books or pictures.
Gist of Idea
Possible worlds could be real as mathematics, propositions, properties, or like books
Source
Joseph Melia (Modality [2003], Ch.6)
Book Reference
Melia,Joseph: 'Modality' [Acumen 2003], p.123
A Reaction
My intuition is that once you go down the road of realism about possible worlds, Lewis's full concrete realism looks at least as attractive as any of these options. You can discuss the 'average man' in an economic theory without realism.