Although it was his ultimate goal, Megatron would never have been content with merely ruling Cybertron. It matters not that their creator intended the Transformers to be a race of peace. The Decepticons are the highest form of life in existence, conquerors who will bring order to a galaxy plagued by disorder and weak organics, which the Autobots seem intent on protecting. Thus, Megatron is destined to lead the Decepticon Empire to rule the universe. He will accept nothing less.
And only one thing is standing in his way.
The Transformers cartoon
By 2006, the Decepticon Empire was essentially extinct. After the Decepticons were driven from Cybertron by Unicron's attack and the disappearance of Galvatron, they fled to Chaar. Without Galvatron, or other strong leaders such as Air Commander Starscream and Military Operations Commander Shockwave (both of the last two were hardly ever seen again), they descended into a period of disunity, spending their time fighting each other for scraps of Energon. Eventually, Cyclonus resolved to locate Galvatron and restore the Decepticon Empire to its former glory. It didn't work out too well.
The Decepticon Empire is not one of conquest, but pillaging. Via the Infiltration protocol they let the planet ruin itself, before taking its resources and letting Sixshot completely reduce it to ash and rubble.
When Megatron obtained the means to send a subspace signal to space, he contacted his fellow Decepticons dispersed among the galaxy informing them to unite in order to raise their glorious empire once more.