More thoughts on the LotFP game, but could very easily be applied to everything I've ever run, maybe baring the 3.5 game. But anything is possible with this.
Imagine a black Sea. It stretches on infinitely. The Sea is rough, harsh, with swells and waves. It has no bottom and no shores. On this raw boiling Sea are boats. These boats vary wildly, but all have an identical function - they dry and keep the contents dry. Some boats are more like submarines, completely sealed from the black waters. Others are open, and the great waves wash over the boats, filling the boat and sinking it. Perhaps only flecks of water splash over, and the contents get a little wet.
The Sea is raw Chaos, the force of magic. The boats are reality, the forces of Law. They contain their own realities. Those who serve Law are the protectors and wardens of the boats, trying to drive away the influence of Chaos where it infiltrates the world. Whether the gods are the higher agents of Law or simply Chaotic beings captured and re-purposed by the force of Law is up for debate.
Magic Users are those who feel the pull of Chaos, of the Outside, of the Other. All Magic Users pierce tiny tiny holes in the sides of the boat, or perhaps stand on deck, trying to bottle some of that Chaos, and using it as raw power to invoke their spells, or perhaps infusing the raw stuff of Chaos into items and tools. Every time a hole is opened a risk is taken, with a larger hole containing greater risk and greater potential power.
There are further implication to this metaphor - travel between the worlds is possible, but incredibly dangerous. The factors involved in this are the distance between the worlds and how open the world is. The more open a world, the more Chaotic it will be. The more difficult, the more Lawful it will be. Staying too long in the Chaos attracts the dire attentions of those who call the seething Sea home, and they will find you. First the small things, those who dwell near the worlds. Tarry too long, and you will invoke the attention of the leviathans from the infinite black below, beings of unimaginable power and destructive potential.
The boat/worlds that sink are entirely overcome with Chaos. They are very easy to visit, being slowly ripped apart by the now free-reign of Chaotic energies. They are terribly unstable places, but entirely unique phenomena can be found there, at the point where Chaos and Law entwine unpredictably. Some of the Chaotic beings will drag away tiny fragments of the worlds and make their own realms, attracting others of their ilk.
There are thousands of splinter worlds which escape notice of the beings of the Sea. If anything survived the destruction of the world contained within, they will cling to these splinters and learn to adapt, as life always does. There may be fleets of splinter worlds within the infinite Sea of Chaos, fighting every day to continue existing, making use of impossible logic enabled by their proximity to the black waters of Chaos, building networks between each other and fashioning craft capable of travel through the Sea.
The completely water-tight realities are sterile wastelands. There is no Chaos, no life, only eternal mineral landscapes where natural processes wind on eternal. Many of these realities will have already suffered head-death and effectively be empty spaces. If one could seek refuge in these worlds, they could be fashioned into safe havens against the Sea.