If we're pitching our alternate Indy IV ideas, here's mine:
Indiana Jones and the Lost Horizon (keep in mind this is with the POD I pitched earlier where this takes the place of Jurassic Park).
The year is 1947. The war is over, but everyone feels like they've been put through the ringer. Yes, the Germans and the Japanese have been defeated, but the revelation of the Axis' crimes against humanity, and looming Soviet threat, make it hard to feel like it was worth it in the end. We find Indy still working as a college professor, but he's definitely gained a few more years, and a lot more mileage. His war-time experience has left him drained and unenthusiastic about his career, and he contemplates quitting archaeology altogether (shades here of
"The Best Years of our Lives"). His father has passed away, he's lost touch with his old friends, he's not doing great.
Suddenly, he's called into a meeting with a few men from the newly-formed CIA. The Civil War is still raging in China, and there have been rumors that a rogue nationalist General (who was rumoured to have collaborated with the Japanese against the Communists) has been looking for some kind of artifact that will let him eliminate the KMT and the PRC, and set himself up as some kind of God-Emperor. No one bought his story until he turned up in Macau selling artifacts on the black market that don't match anything on record. Finally, the CIA spooks drop the bombshell: there's rumors he's found the path to "Shambala", the mythical Tibetan city. The CIA, looking to preserve the balance of power in Asia (foreshadowing the coming Korean War) want Indy to stop him.
We follow Indy from Macau, up to Mongolia (maybe track down some lost Genghis Khan stuff), then through the Chinese heartland (where we see the aftereffects of the Sino-Japanese War, and the ongoing Chinese Civil War), all the way to the Tibetan plateau and the Himalayas. Indy beats the General to Shambala, meets with the High Llama that runs the place, and is able to achieve a some kind inner peace. In the end, Indy destroys the entrance to Shambala (wiping out the Warlord's forces in the process) to protect it from the outside world, but he leaves knowing that the "worlds is a fine place, and worth fighting for).
In the end, he winds up on Marian's doorstep, and (after some flirty shit-talking) she invites him in for "a drink", hinting that they'll rekindle their relationship.