The original film was set in 1936. If they set the movie 40 years after the first one, Indy should be doing his thing about the time of the Bicentennial of the Signing of the Declaration of Independence.
Maybe he could meet up with Ben Gates? Indiana Jones and the Galactic Treasure? Or, if it’s a prequel, Indiana Jones and the State Treasure?
IJ#5: “Indiana Jones and the Declarative Case”