The Astral Sea
The Astral Sea is an infinite expanse outside of time and the traditional planes of the multiverse. It is the fabric into which all other planes are woven, and where the majority of gods reside.
It operates beyond the normal laws of physics and magic, and exists--in some way--in the mind of everyone who observes it. Most mortal beings will perceive it as an expansive ocean filled with stars and unknowable depths, with islands of all shapes and sizes scattered at apparent random.