When I watched London, May 1916 yesterday, I thought about the famous language scene and a couple others and realised you can see what countries Indy visited and therefore what episodes could have been made.
ok, first they speak french (Paris, September 1908), then german (Vienna, November 1908), then italian (Florence, May 1908), hungarian (no episode I could think of), swedish (a Stockholm episode was planned), greek (Travels with Father) and Arabic (where did he learn that?). Indy also mentions his trip to China once, when he and Vicky meet the poor woman and her children.
Le Havre (a planned episode) was mentioned right at the end, when Indy talks to Miss Seymour.
A lot of episodes actually foreshadow events in later and planned episodes. In Palestine Indy talks about his trip to Australia for a second (plans for that episode were confirmed) and in the bookends of Travels with Father, we find out about Paul Robeson and Indy being friends (Princeton, 1905 was planned). Curse of the Jackal foreshadowed Indys affair with Mata Hari (when he and his cousin see her picture in the magazine).
There are several more and I hope you'll help me find them.