Dragonstar looks awesome, but its a fusion. I'm thinking more Spelljammer with Scifi stuff. I've worked up an idea that I think can do it though. So the Phlogiston is a nebula containing the Crystal Spheres which each, in turn, contain a fantasy system, in addition there are "Free Systems" which float in the Phlogiston but cannot see it due to their stars rays. At the core of the Phlogiston is a "Prismatic sphere" around which the "core spheres" of Mystaraspace, greyspace, KrynneSpace, and Realmspace orbit. the Phlogiston seems to eminate from the Prismatic Sphere. In the Phlogiston is quite a bit of Flotsam and Jetsam (Like the Warden). Early (50's - 80's) mutations are caused by Phlogiston radiation, so some of the "Free Systems" are full of Mutants and Masterminds style mutants.