Suppose a "TdF" was run with all teams required to ride 9 man tandems. IT would be very different. On the flat, really fast! Drafting would be close to meaningless. Climbs would be hard.
I would propose that all teams must finish with their 9 man bike and empty seats for all riders who could not finish (or start) a stage, same as current. Choosing riders for your team would be different. I suspect the value of the hard men of the classics would rise. A 120 pound rider on 1500 pounds of bike and teammates would have little value anywhere, even the mountains unless the whole team was that light.
I know, just a pipe dream. but fun to think about.