The thinking and prototype which could lead to helicopters before aircraft actually existed, in the form of a child's toy.
Take a light disk , with radial vanes on it, pitched. mount it loosely on a platform, which itself rotates on a vertical handle when a string is pulled. When the platform spins lugs on it will spin the disk which is not otherwise fastened to the platform.
Pull the string quickly, the platform and disk spin rapidly, the disk will shoot up into the air to a surprising height.
I had one of these as a child in the 1930s, well before the invention of the helicopter, and I have seen advertisements for them in Victorian times (I think modern versions are still for sale)
If a suitably brilliant mind saw such a toy, and imagined a large scale version, you are a good way down to road to the helicopter.But, I think the unsolvable problem will be a light enough yet powerful enough power plant. I do not think that is possible.