My vision for the end of SG-1 (ending after season 10) and the beginning of the new, third series:
With the whole Milky Way fallen to the Ori, only Earth remains as a free planet. The Ori have avoided it up until this point because of the possibility that Ancient weapons could be used against them. However, finally the Ori fleet, now 20+ ships strong, heads towards Earth. With only 3 BC-304s in orbit and the Ancient defense platform in Antarctica barely working, Earth will not be able to defend itself. Only two days remain before the Ori fleet arrives at Earth.
The president finally decides to announce the Stargate program to the world, having no other choice at this point. An outcry comes from the rest of the world as the US President makes his speech. The President sees no way to defeat to Ori and announces on American television an evacuation plan. He plans to send as many people through the Stargate to Atlantis as possible. The Stargate will be loaded onto a plane and brought to an area just outside New York City so that the greatest number of people can be saved.
When the rest of the world hears about this plan, the Russians and Chinese, among many others, demand use of the Stargate as well. The Russians especially, being owners of the gate, demand its immediate return to them. The US, of course, refuses. In an act of spite, the Russians and Chinese launch missiles at New York in an effort to stop the evacuation plan.
With the world in chaos, rioting in the western US and missiles inbound, millions of people in the eastern US get in their cars and head for New York. Using the Asgard beams aboard the BC-304s, the missiles are stopped well before New York. The evacuation becomes a complete mess with rioting, looting and tons of violence as people fight to get to the Stargate. Throughout the course of the day and a half of evacuation, only a few hundred thousand people are saved. The only other people saved in this time are the world's greatest scientists, doctors, thinkers, etc, as they are all beamed onto the orbiting 304s.
As the Ori ships finally burst from hyperspace on the fringe of our solar system, the 304s leave for Atlantis, taking with them the Stargate and any evidence of where they had gone. The Ori ships are greeted by the full nuclear arsenal of Earth but this only serves to rattle the shields of a few of the motherships. As the Ori fighters descend on Earth, air battles begin all over the world between Earth fighters and Ori raiders. The Earth air forces are quickly subdued and the Ori begin decimating some of the larger cities from orbit. As the Ori ships land, their forces spread out and begin to defeat the ground troops that the various nations had gathered to fight them. Within four days the Ori are in complete control. They soon control all of the Earth and begin the forced prostration to the Ori. In a matter of months, the Tau'ri have been stripped of all their technology, they are forced to live simple lives and all guerilla efforts are snuffed out by the Priors. As time goes on, the resolve of those who remain dies out and the Earth becomes another planet living only to serve the Ori.
Cue the new Stargate series:
300 years later, Atlantis has become a fully occupied city again. With a population of over two million now between the city and the mainland (now mostly industrialized), mostly the descendants of Earth survivors, almost all of the secrets of the city have been unlocked and the city is operating with three fully-powered ZPMs. The Wraith and the Asusrans have been defeated long ago and the Neo-Atlanteans are the unquestioned superpower of the Pegasus Galaxy. Over the past 300 years, massive amounts of research and building have been undertaken to build an enormous and very powerful fleet. Many secrets of the Ancient database have been put into use in these ships. The production of ZPMs, drones and even Stargates is no longer a mystery to the humans of Atlantis. The driving force behind all their efforts in rebuilding has been the idea that one day they would build a big enough fleet to lead a crusade back to Earth and retake their old planet and the rest of the Milky Way. The series could show the war against the Ori and also have tons of flashbacks to fill in the gaps between the time of the evactuation from earth until the present time in the series, showing how they built their empire and brought atlantis back to being a fully operating city
Okay guys, what do you think? *respect for anyone who actually read this far*