I've been thinking of a fun way to combine campaign worlds together, and came up with something I think works well. The concept is to justify having a patchwork campaign setting, wherein you could drop a location from any campaign world without upsetting balance in the world or throwing the suspension of disbelief.
Three important rules govern how this setting exists:
The first rule is that the material plane is a result of other planes overlapping. Ice and fire, light and darkness, order and chaos -- all of these things and more have their own respective planes of existence, and where they converge on the cosmic wheel, the world is formed.
But the world does not fold neatly into itself. In some places, one plane of existence dominates over the others. This phenomenon creates places like Sharn, the City of Towers, where impossibly tall buildings exist solely because they are constructed in an area that is predominantly elemental air. Other examples might include Ravenloft, a horrifying realm of darkness, or the Tablelands, a broken wasteland of baked earth.
In these areas, spells act differently according to whichever element dominates. Similarly-aligned spells will be augmented, while opposite-aligned spells will be dampened.
The second rule is borrowed from Ravenloft, which introduced the concept of "the mist," an ethereal void that surrounds the campaign world. People and places sometimes appear from the mist without warning or explanation, perhaps displaced from some other world or created by the strange nature of that place.
This is the central concept of the game world. Because of this phenomenon, areas of the prime material plane frequently become displaced by fragments of other worlds; however, these areas are almost always surrounded by the mist. Crossing this barrier is dangerous and survival is never guaranteed, but with some luck or the help of clairvoyants known as mistwalkers, it is possible to ferry groups between the worlds safely.
The third rule is that the world is a fabric, and magic is the loose string. The very act of using magic is, in fact, the process of unmaking reality. The more you pull on the loose string, the more you tear the world apart.
Although the displacement effect has led to the discovery of new civilizations, and thereby new technologies and magics, such areas tend only to appear wherever overuse of magic (such as that used in mass warfare) has caused the fabric of reality to tear open, creating converging dimensions (and perhaps even timelines).