Space Elevators
I have been thinking about this one and I keep on coming back to the problem of it being a really big target. Basically the only way for this to work is on a relatively peaceful world. My Fusion Age game is anything but peaceful. I like the idea but I think that it would get literally shot down.
Basically I am basing this partially on the Eugenics Wars referred to in Star Trek. Transhumanism is strong in various forms. Some people have converted themselves into a cyberbrain with various cybershell bodies. Others have gone the route of genetic augmentation. These Augments are setting up various kingdoms on Mars, under the Earth Oceans, and along the Main Belt. The Earth Alliance is trying to deal with all of this right after the Fall of Earth (basically a Nuclear Winter on Earth set off by religious fanatics causing a major space station to crash on Earth).
The Tech level is called Fusion Age because it is defined by easy Fusion energy because small table sized Fusion reactors become common. Underwater Supercavitation Submarines and Subfighters, Fusion Torch Starships, Land Mecha with Thermoptic Camoflage, and heavy cyborgs become the norm.
Earth is under a Nuclear Ice Age so that the oceans are the only safe place to be. Venus is slowly being terraformed. Mars houses the refugees from Earth and is in constant warfare over the few resources and a hundred old hatreds. Mechs fight in the deserts of Mars. The Main Belt is the home of Megacorps producing Starships for both sides of most conflicts. Babylon 5 style space stations float through the Belt as the main places to relax in the Belt.
Sound like fun?