I'm with you on that one.Well, since all four movies has a supernatural element, they'd have to do the same, or the audience will get bored (I know I would). An artifact is one thing, but having it attached to something out of this world is what put IJ into greatness.
The Idol, the Cross of Coronado, the Lao Diamond were neat and make good action sequences, but you can't make a movie out of them.
Perhaps you could introduce the mysteque of lesser know objects by having a local person from the area, recount tales of it's past supernatural actions. Perhaps a slight deviation away from the Indy format rules, where we see a flashback to a pervious time intercut with an Indy intro. We assume they are occuring similtaniously, but there is a reveal where we see they are seperated by several 100 years.