Spell Jammer and the Crystalsphere of Kryn From Dragonlance

The spell jamming helm creates an atmosphere that you can breathe. So plainscape unifies all Dungeons and Dragons campaign settings because they all feed into it, right? Spell jammer did that first. And by basically saying that every campaign setting is its own prime material world surrounded with a crystal sphere in the Potolomaic astronomical system. There's a crystal sphere around your little local bit of space and those crystal spheres are all connected by a foggy pink magic thing called flogiston.