First, I'd give it a shorter title. Much shorter than "Kingdom of the Crystal Skull". Maybe "Indiana Jones and the Forbidden Treasure" or "Indiana Jones and the Lost Talisman".
Second, I'd get rid of Mutt right away. Then give Marion a
much bigger role.
Third, I'd have the film tie back to
Raiders of the Lost Ark, as Indiana Jones is forced to unravel one of the deepest mysteries he's ever faced - the whereabouts of one Abner Ravenwood. Because, naturally, he's not dead.
My original idea was to have him be captured by the Nazis but, that's been done in
Crusade. So, instead, Abner Ravenwood is working for/with the Nazis, the ones hiding in South America post-Nuremberg. They're looking for something, be it Atlantis, Excalibur, Mjolnir, Palladium... a weapon powerful enough to start a Fourth Reich. And it's up to Indiana Jones to stop them.
Fourth, there needs to be an original vehicle set-piece. Raiders had the truck chase, Temple had the mine-cart chase, Crusade had the tank fight. Kingdom just had another truck chase, which was a bit tame, really. I'm not sure what I'd use here: trains, nuclear submarines, giant lorries, hovercrafts, whatever. But that'd be there.
Fifth. The bad guy. Obviously, it's the Nazis. But there's also Abner, who's sort of like Belloq, only older. You gotta cast somebody truly magnificent to play that role - I'm feeling Donald Sutherland, or somebody similar.
And finally, I'd skip the Lucas dialogue. "I never should have doubted you, old friend." Ugh.
And that would be my fifth Indiana Jones movie.
Thoughts, comments?