Here's what I've got:
Humans have abused the Earth's resources to the point where global warming has fully melted the ice caps, and places with large amounts of snow and ice in the world. This of course leads to mass flooding of Earth, with an estimated 80% of the earth's original land mass being underwater, or uninhabitable due to the water making infrastructure, etc useless.
There are 6 cities, dubbed - "Megacities" which contain altogether around 3 billion people, and these cities are built on what's left of major cities that haven't been flooded yet. The poor, or just plain unlucky people remain living in these megacities, while the rich live in space stations and moon colonies outside of Earth. The megacities only have an estimated 5 years left before they crumble due to the Armsden Industries (The world's government and leading business in all aspects, think of Weyland Industries from Aliens) planning on leaving the poor and unfortunate behind and taking the educated and/or the rich to other worlds and civilizations.
1 year before the Earth goes into anarchy, the first official civilization is established in the upper atmosphere of Venus, this floating city is called Empyrean. Only the rich and educated live here. Watch
this video to see why I chose the upper atmosphere of Venus.
1 year later.
The story revolves around a prisoner called Gaius, who is an anarchist (or terrorist, depends how you look at it) who has his own independent goal of taking down the corrupt, fascist government who control everything in the Sol Solar System (our solar system) under military standards.
Gaius is in prison for acts of terrorism, on an extraordinary scale (you can come up with this, or ask me for some ideas), and just before the government are about to leave Earth to it's own devices, they contact Gaius, and ask him to come with them to Empyrean to develop WMDs for Armsden Industries, with the intention of using it against hostile civilizations far away from Sol.
Now, I have a few plot twists you could look at here:
- The carrier ship transporting Gaius to Empyrean is boarded by anarchist ships, and he gets captured by them
- They imprison Gaius on Empyrean, and he has to escape the prison (think Riddick)
Or they keep their word and employ him as a WMD developer on Empyrean.
This is basically taken from my own writing here, but feel free to use it or take inspiration from it.