In my opinion Lucas was always full of ideas but he didn't know where to stop. I always found the story of Temple of Doom's development funny where he came up with the following, later abandoned ideas:
- Motorcycle chase on the Great Wall of China
- Hidden valley inhabited by dinousaurs
- The Chinese Monkey King
- Haunted house in Scotland
- Tiger hunt
- People drinking magic blood and turning into superhuman zombies
The following ideas made it into the film, IMO they are not any less weird than the others: - Musical opening number
- Demonic temple in India
- Religious cult
- Black magic
- Human sacrifice
When developing The Last Crusade, he again came up with
- the haunted mansion in Scotland
- Indy battling ghosts and demons
- the Chinese Monkey King resurrecting him from death
- finding the Fountain of Youth in Africa, or the Garden of Immortal Peaches
- pirates
- Nazis with robotic arms
- Henry Jones Sr. ascending to heaven
And he was already toying with Indy and aliens in a script in the 1990s, which already included nuking the fridge. (I've read a funny origin to the nuked fridge element somewhere but I cannot find it now.)
He's basically a big kid.