For example, can Superman just sit in stars and become infinitely stronger? Can Flash do the same via the speed force? Is there a willpower cap to Green Lantern and his ring?
I know Flash can absorb speed from the Speed Force, but he does have weaknesses. Like his hyper-metabolism. If he doesn’t eat an absurd amount of food to replenish his energy, he’ll run out of energy.
Not sure about GL, but I really don’t know what more willpower would do besides keep his constructs up longer or make them stronger. That’s also an internal issue, so it probably depends.
I don’t really know how Superman’s powers would be affected by his distance to the sun, but if anyone does, I’m sure it’s @HubCityQuestion.
There is no upper limit to Superman’s power, as explored in the works of Grant Morrison specifically, including DC One Million, All-Star Superman, and Final Crisis. Superman could potentially spend hundreds of thousands of years in the heart of the sun, absorbing its power, and then unleash it accordingly.
And that’s why Batman has kryptonite!
Depends on the creators really.
We don’t ask questions like that anymore