A few centuries after the first interstellar mission, the solar system will become overpopulated because the emigration to other systems would be too low. It would be technically impossible for the population surplus to emigrate in solar sails because it would be too expensive. It would be about as expensive as a lift to orbit is today and thus would not be the solution for a massive emigration. Unless a better mean of transport will be invented, the population surplus would have to remain in the system.
In order to make place for the population surplus, we could build megastructures. They would resemble the Stanford torus but would be much bigger.
Eventually huge orbital rings of about three milion kilometers of diameter would be built. They would spin to produce artificial gravity in their interior surface and would be sized so that the rate of rotation necessary to produce a comfortable gravity level would be approximately equal to one day. Since the ring would be tilted on its rotation axis and would orbit the Sun, a day-night cycle would be experienced by the inhabitants. High walls would be built at the edges of the rings preventing the atmosphere from escaping.
The construction of the smaller rings would be technically possible with the use of the same materials as for the space elevator due to their high resistance. But for the larger ones more resistant materials would have to be found. Nevertheless, such megastructures are still possible in theory.
The orbital rings concept was based on the science-fiction series by Iain M. Banks.