I say this a lot.

I still think the 4th Indiana Jones movie would have been far better if...

Indy's ready to pack it in. He's lost his job. His father's dead, Marcus Brody's dead. The radiation he's just been exposed to is slowly killing him though he doesn't know it yet.

Suddenly, instead of Mutt Williams, this young, enthusiastic Egyptian grad student shows up on his doorstep. It's one of Sallah's kids! One of the ones from the first movie! The kid spent all childhood having Uncle Indy show up with tales of lost relics and forgotten kingdoms, and has a freshly-minted Oxford/Cambridge/Harvard/etc degree, and now he's on the trail of the crystal skull.

Hilarity ensues because here's this very sincere young man who wants to do everything by the much more careful standards of archaeology that have developed since the 1930s, and he's in the middle of this two-fisted pulp adventure with his childhood idol, who turns out to be about as methodical as Heinrich Schliemann only now with more punching people. Oh sure, he's good in a fight or a car chase, even if he does constantly apologize for punching people or cutting them off in traffic, but he's a fish out of water. By the end of the movie, though, he's easily as swashbuckley as Indy is, and when Indy gets hitched, he settles his own fedora on the kid's head.

This, and having mummy bundles and more discussion of Nazca lines, is why I think I'd write a better screenplay.