It is said that as the tide began to turn in WWII, a Japanese General named Yamas*h*i*t*a hid loot that had been stolen from China, Korea, Philippines, India, etc. in a small series of caverns and caves somewhere in the most remote areas of the Philippines, both out of spite and for possible retrieval if he survived the war. Some say these caves are just a huge hoax, since it would have made more sense to hide the loot in Japan or possibly Southeast China. Some say that it was true, but that gangsters and crooked politicians long ago looted it. Some say that to this day, Yamas*h*i*ta's gold lays undiscovered in the Filipino rain forest....
My idea for plot (written in a way not to reveal KOTCS spoilers): A Chinese Agent who Indy and Mac worked with during WWII (okay, that WAS a KOTCS spoiler, but nothing you wouldn't have known already) comes calling. The agent is now for Communist (booooooo) China and the Commies ostensibly need the treasure to keep themselves from going broke in the wake of the disastrous "great leap forward". The real reason, since it would be basically impossible for the Chinese to move so much stuff in secret, is because there are artifacts amongst the loot that would help them TAKE OVER THE WORLD (MWAHAHAHAHAHAHAHA). Indy and Friends have to stop them.