How long before the 'hero' runs into their opposite number - someone who goes from world to world slaughtering the local heroes, maybe for the power, maybe just for the thrill of the hunt, but definitely not with any notion of helping their fellow villains?
Shortly after that, the two of them would be hunting each other across all the worlds that be. The collateral damage would be high, and the victories temporary. Transfictionality gives both sides too many ways of cheating death.
And then there's the armaments. The black hat can collect the most potent evil artifacts and superweapons from every fiction - the One Ring, Stormbringer, the death star - so the white hat will have to do likewise - Excalibur, the Ginzuishou, a war-Tardis. (Even if the 'hero' would rather avoid a fight, the villain would be unlikely to want potential competition on their territory.)
This would be a difficult story to tell without looking ridiculous, but done right it could be great.
Shortly after that, the two of them would be hunting each other across all the worlds that be. The collateral damage would be high, and the victories temporary. Transfictionality gives both sides too many ways of cheating death.
And then there's the armaments. The black hat can collect the most potent evil artifacts and superweapons from every fiction - the One Ring, Stormbringer, the death star - so the white hat will have to do likewise - Excalibur, the Ginzuishou, a war-Tardis. (Even if the 'hero' would rather avoid a fight, the villain would be unlikely to want potential competition on their territory.)
This would be a difficult story to tell without looking ridiculous, but done right it could be great.