Ever since I started thinking about the old TF2 characters from the picture in the Engineer comic, I've thought about a game mode (possibly mod?) where every characters abilities is stripped down a lot with way smaller amounts of ammo.
Like instead of designing every class to be capable of fighting for themselves, making them more interdependent.
An example would be making it so Demoman only has a sticky bomb launcher and nothing else, effectively limiting his role to territory control and nothing else.
Like hyper-defining every classes role. Maybe make Scout extremely weak but also make him cap points very, very fast. (with a class limit on how many there are).
Not sure it would be fun so much as interesting to see what comes out of it. I know its drastically different than how TF2 currently works.
Another idea would be to make pyro's flames extremely powerful, but give them very low hp and take away every other weapon.
Basically, making everyone way weaker so that games end up defined more around group skirmishes than general death match style.