I'd have Earth as in our planet as a world that you can travel to, it's modern day earth much like ours, a world that's more technologically advanced than most worlds...
Except here's the thing about it, it's the Earth that's the setting of Masque of the Red Death, Dark Matter and Urban Arcana all at once...
What happened was the Red Death was defeated some time during World War I, and this cause massive breaches in reality and the planar fabric of our world. The outside of Earth's reality became known as the Shadow, and could warp the minds of those from Earth, masking the presence of otherworldly forces and residents. With the Red Death gone, various forces from other worlds found it easier to come and go from Earth. Humanity of course didn't need the Red Death's influence to do any of the atrocities that followed its banishment. But it started a secret conflict from where various forces from other worlds would come into conflict with each other, and the various Illuminatis formed indirectly from the Qabals.
Come to the modern age a force that the Hoffman Institute has labelled as Dark Matter is coming towards Earth... However those in some of the outerworldly factions and the former Qabals might have a better idea of what Dark Matter really is...
Now many refugees and travellers from worlds beyond the shadow, and their descendants live on Earth, in small communities hidden from the minds and eyes of most mortals. Many live their lives as most people of Earth do, and many don't either planning their escape from our world (somewhat difficult but not exactly impossible to do, and mostly the matter of finding a portal), hiding away biding their time, or working for the interest of extraplanar forces.
Also certain groups native to Earth (including some Fraal who are native to our reality) are aware of the natives and forces from the Shadow. And many do take an interest in them as well...