There is no need to make another Indy movie, The Last Crusade they ride off in the sunset, and that is a perfect ending to a good trilogy. The first one is a classic, the other two are good sequels, and thats it.
If I go by anything of the PT and Godfather III, is when you make movies of a series 15 years apart, it never works, it's just too much time, and the director usually changes for the worst. Coppola, Lucas, DePalma, guys like this lose their drive that they have as young directors, and don't have the hunger they did when they made their classics in the 70's. Is it a coincidence that the best films of Lucas & Coppola are their first few efforts?
Sometimes when they are up again huge odds to finish a movie, or don't have the budget they would like, just as Coppola was about to get fired making The Godfather, and how he had a heart attack trying to finish Apocalypse Now, and how Lucas almost had a nervous breakdown shooting the original SW, and Spielberg couldn't get the shark to work in Jaws, that is why those movies are great. Now with their unlimited budgets, and secure lifestyle, and raising kids makes that director lose that edge.
Sure Spielberg still is great, and of course he would direct Indy IV, so I know it won't suck like TPM did, but it still isn't needed, I had the memories of SW ending on Endor, and that was ruined by the PT, let me remember Indy by Harrision Ford & Connery riding off in the sunset. Come on George, don't ruin this trilogy too! You had SW backlash, do you want a whole slew of Indy fans to hate you now?