The problem with one more Indy film with Ford is that it would have to be set in the 60s or early 70s to allow for his age now. And that is just not Indy territory any more. Indy should be set in the classic serial era and not go beyond, say, 1945. I'd even argue it should avoid WW II itself. Indy's world is one where magic - of a kind, anyway - is still a possibility; one where a certain naive sense of wonder is still prevalent. I feel that doesn't really mix well with WWII, with the Cold War and most of post-modern history. You can set a roaring adventure in pretty much any time, of course also in today's world. But it's simply not the same, no matter how hard you try to adapt the Indy atmosphere to the jet-age. That's why Raiders of the Lost Ark is a classic now, while The Jewel of the Nile is just another 80s popcorn flick.
Spot on observation!