I always wondered what happened to the humans/Earth prior to the Beast Wars era (300 years after the Great War ended) so I came up with some ideas.
As the war between Autobots and Decepticons raged on, with Earth still caught in the crossfire and more bots and cons than ever requiring energy for fuel (and getting larger/more power-consuming all the time thanks to all the new tech like Powermasters, Targetmasters and Headmasters), the nations of the Earth secretly developed a new piece of technology, and when ready, held a secret meeting with Optimus Prime to inform him of what they planned to do.
The nations of the world would broadcast a message simultaneously across every form of public communication that all Cybertronians were to leave Earth immediately and never return, otherwise they would face immediate termination.
The broadcast explained that top scientists from around the world had developed a powerful wave generator whose energy would instantly short out permanently any Energon based technology, bet it Autobot, Decepticon or any other and that this device had a range that would cover the entire solar system and that once turned on, it would remain on indefinitely, hence any Cybertronians who attempt to return to Earth would be terminated upon entering the solar system.
Forced back out into space, the Autobots and Decepticons agreed to a temporary ceasefire and to allow each other safe passage to their homeworld of Cybertron through a newly build Space Bridge (which would be destroyed by the humans once all Cybertronians were offworld).
Upon reaching Cybertron, Optimus Prime attempted to negotiate a lasting peace with Galvatron and the Decepticons, but it failed, resulting in a large scale war for control of Cybertron, much like the first war that resulted in the Autobots finding Earth.
This time however, neither side would yield control of their homeworld in order to seek out resources elsewhere. Both sides were tired of the wars that had raged on between them for eons, and regardless of the outcome, this would be the end.
After years of fighting, Cybertron's resources from its second 'Golden Age' now depleted by battle, and Galvatron destroyed at the hands of the now dying Optimus Prime, the remaining Autobots, of which there were not many, negotiate a pact with the Decepticons who were also low in number after all the fighting, one which would allow the war to end and both sides to remain on their homeworld and survive, even with the limited resources which remained.
They would use Vector Sigma to create new, smaller, more energy-efficient forms. They would be allowed to retain their existing allegiences under the treaty, and each side would have representatives in their new government. This was not optional, and any Autobot or Decepticon who refused the conversion procedure would be exiled from the planet forever (needless to say, no one refused).
The newly converted Autobots became the Maximals, and the Decepticons became the Predacons.
They decided on semi-democratic system of government where each side would be represented by a group of leaders who would negotiate terms and laws with one another and would be responsible for keeping their own sides in check.
For the Maximals, it was the Elders, and for the Predacons, it was the Alliance.
Thus the Pax Cybertronia was chartered, which stipulated, among other things, that while both sides were permitted to seek out resources elsewhere and to explore space, they were not to come into conflict with any sentient species on any world and were to keep their very existence a secret by using their scanning and replication technology to blend in with the native life on explored worlds as best they could.
It was also agreed that no one was allowed to attempt to return to Earth for any reason, and that all alliances and ties to that world were now severed as they'd agreed with the human leaders there before being exiled to Cybertron.
I am Alpha and Omega, the beginning and the end. I am that which is, which was, and is yet to come.
And you will know my name is MEGATRON when I lay my vengeance upon you!