It would be pretty cool if Indy 5 took place in the year 1981 (the year when Raiders came out)

The character of Indiana Jones was born in 1899, so he would be close to Harrison Ford's actual age in my proposed timeline of the movie.

How could you make this movie work with an 80+ year old Indiana Jones? A simple solution would be to have the older Indy appear at the beginning and the end to bookend the movie. Perhaps he is giving a speech at a museum where he is being honored for his life's work and then there are foreign agents who threaten to kill him and the museum's curators for the macguffin that happens to be in the vicinity.

And just as Indy finds himself that he is about to die, his mind flashes back to the time when he was in an adventure to retrieve the macguffin in question. I think an appropriate time for this would be in World War II--which has never been depicted in the franchise. If Harrison Ford would allow it, he could have makeup and CGI to de-age him to make him look like he is in his 40s.

And once the flashback (which comprises the middle 80% of the movie) ends, the much older 1981 Indy finds a way to survive and save the day.