Found this post while digging to the bottom of the internet. September 1993:
And here's a murmur from 1994.the script reportedly, originates from the Young Indy show...
What would need to be changed are the dates (obviously) and some settings (i.e. adding Nazi's). But the basic essence of the story (i.e. the object Indy has to discover) and the research that entails would have already been done. Put it this way, Lucas (according to USA Today) recognized that one of his scripts had alot of potential, and that's when he sought out Spielberg and Ford.
I first heard about Harrison Ford's interest in another Indy movie back in early August on the radio.