Well they came out with a couple different Zwarte Piet sets, Netherlands is, what, 20 million? So that would be the lower bound as far as country size. Other holiday traditions cover many countries, as the perennial Christmas theme shows. Easter, New Years, Midsummer - there are a few more holidays that would have broad appeal.
The museum sets are as I understand being driven entirely by museums, so as long as PM makes a little dough on it they'll do even obscure figures like Kneipp. They haven't done any for non-German museums apart from the Rijksmuseum, but maybe they just haven't been asked yet.
I think the possibilities are almost endless.