THE ARCANE EMPIRE
The Empire is only a few thousand years old forming after a planet of magic was invaded by a planet with advanced technology. In the end the dragons came to rule and the two major pantheons of dragons (metallic and chromatic) were the ruling class. All other species are expected to be loyal subjects to their Draconian masters paying tribute and obeying the mandates handed down. Within the ranks of the dragons there is constant political maneuvering and infighting, even assassination and attempted coups, but most of the galaxy is made up of the subjected beings.
The truth is controlling every being in the galaxy isn't a small task and even the dragons of the Empire aren't really able to do it. Many planets are still primitive and there's no way to force wide scale compliance, and even in the more advanced societies many still escape notice. The usual practice is to move into the system, blast their major power centers into submission, recruit a bunch of new legionnaires, drop off a regiment to collect taxes and ensure they planet doesn't rebel, and move on to the next system.
On Earth the war lasted around five years, but in spite of the resistance put up the governments of the Earth were forced to accept defeat. Many of the soldiers from other worlds ended up settling on Earth leaving large, but minority populations of Elves, Dwarves, Orcs, and other humanoids as well as a host of other things that came along with them. Naturally not everything about being invaded by a technologically and magically superior force was bad since as subjects of the Arcane Empire we now have access to magic and technology surpassing what we had before the invasion. Many people from Earth were conscripted into service and are now spread around the galaxy and others have learned to travel on their own from what the Empire brought to Earth.
Life as a subject of the Empire can seem similar in many ways to life before the invasion if you live away from hot spots. Military service pressed or not is a desirable option for many, only after 5 years of service in the Imperial Legion does a person gain citizenship (even if born into the empire), the importance of citizenship is it is the only thing granting rights in the Empire. A non-citizen may go about daily life just like anyone else, but if eaten or taken as a slave they have no legal protection. In addition to doing 5 year in the service everyone is expected to pay their taxes which is essentially 50% of everything you make (even if you literally make it by hand), although again enforcement is much easier in more central locations and on higher tech worlds. Local jurisdictions are allowed to impose tariffs on the use of services or even sales taxes, but property and income taxes aren't allowed.
This setting isn't meant to be particularly far into the future, more like a cyberpunk campaign it might only be ten or twenty years from when the DM is running the game (should be at least 5 years after the invasion so characters can have been through their service period to the Empire).
The action can take place on Earth with missions and encounters much like a shadowrun or dark conspiracy type game or the characters may go to other worlds in search of adventure or wealth with settings like the classic D&D worlds of Toril and Greyhawk or the Pathfinder world of Golarin and for something more exotic the Talislanta world. The general assumption is that the Empire doesn't have cybernetics or gravity based technology; if a DM wants to introduce that level of technology it could be done with Pathfinder's technology guide, although magic can usually accomplish the same goals.
Most things from D20 sources should convert easily, this system doesn't use feats or metamagic so items and powers related to those can be ignored.