With the new boiler system, it would be cool if there were a geothermal heater for large boilers and furnaces. Also, another idea that came to mind... The sun gives off a ton of radiation, not just light, which can also be used for power. Theoretically, an even more efficient form of solar power would be setting up a giant metal mesh and using it as an antenna to capture the charged particles coming off the sun... Of course, the antenna would need to be deployed beyond the earth's magnetosphere to work, since the magnetosphere is why life can even exist without being irradiated to hell by those charged particles. In-game we could ignore this and set up the antenna right below the height limit... Or it could be a type of satellite that can beam the power back down via a microwave laser and get collected on the surface.