I want to make it known that I’m not advocating for Carousel of Progress to remain in Magic Kingdom and stagnate. It absolutely needs some love.
To my mind, there are three options (all would involve sprucing up the existing scenes included, but otherwise not really changing them):
1. Keep the first three scenes and re-do the final scene to better reflect 21st century technological advancements.
2. Keep the 1900s scene, make the 1940s scene the second scene, create a new third scene for the 1980s, and create a new final scene for the 2020s. An “every 40 years” sort of passage of time could work quite well.
3. Keep the first three scenes, and make the final scene set in the 1960s. This is obviously the most “retro” option, but it would be in keeping with the current “every 20 years” narrative, and could pay tribute to the ‘60s “gee whiz” optimism/futurism that informed so much of the company’s output in that era.
I would personally go with option 2. Half the classic experience, half a new one.