Well, it turns out that great minds think alike (or fools never differ). At least one blogger posted his or her own synopsis for the hypothetical Indiana Jones and the Spear of Destiny. On another site, a fan wrote a partial script with the same title (along with other fan scripts such as the rather polemical Indiana Jones and the Blade of Abraham) A recent discussion of the upcoming film, New "Indiana Jones"? also suggests using the Spear of Destiny. Basically, everyone and their mom is thinking along the same lines.
The surprising thing (or not so surprising to some) is that all of us are horribly slow and Indiana Jones and the Spear of Destiny already exists... as a comic book and it was even almost made into a video game. I'll try to check it out the next time I walk into a comic book store.
But I'm still thinking that a film version of Indiana Jones and the Spear of Destiny could be done (or re-done) in a powerful and effective way. Perhaps Jones could get the Spear from the Nazis and he turns it over to the US government towards the end of WWII, around the time of Hiroshima, and the film could end suggesting that we still have it today. (damn! that could be deep) Does anyone out there have other suggestions for what the fourth Indiana Jones movie could be about? Finding the Garden of Eden? the True Cross? Noah's Ark?