Although solar during the day and gas at night can be done, it's tough sell economically since two plants are being built and run instead of just one. Once the tech is there, I'm sure it'll be done, but economics won't be the driving force.
Water scarcity is another up-and-coming problem, with many important aquifers around the world being close to running dry from being depleted faster than they can regenerate for decades. A lot of water pipelines have been built lately, some across entire countries. A lot of countries are have also been experiencing 1-in-250 to 1-in-750 year droughts or floods over the last decade or two, which complicates water supply issues.
And why is Theo dragging this issue into this discussion?
Most future power plants require large amounts of water. Nuclear, solar thermal systems, they all require water. And with less available fresh water worldwide, pumping water from farther and farther away or using desalination plants both require lots of power. So what I am trying to say is that, even if the population was not growing, power use would still grow exponentially. Finding cheap and reliable power will become a much bigger issue for the next generation. And for some areas, water.