Okay so we know you like the idea of Krass being rock person or hybrid rock person but how would you do Subternia in Core?
Subternia being in the Core with the Star Seed?
Or, how would I do my version of Subternia, and my version of the Core where the Star Seed resides?
Subternia: To start, I'm not so fond of Revelation's interpretation of Subternia just being Eternia's "hell" or "Tartarus". Its not a part of the afterlife in my headcanon. In my headcanon, Subternia is a system of underground caves deep under Eternia's crust, somewhere between the surface and the Core. This system of caves also connects to the subterranean lake beneath the Evergreen Forest, which means there's also an access point into Castle Grayskull in Subternia.
the Core and Star Seed: Honestly, no interpretation of Eternia's Core and the chamber of the Star Seed will ever be better developed or more dynamic than @skeletorboyfriend 's own personal fanon of "Coreternia". I highly recommend hopping over to their blog and reading some of their info posts about it. He put more effort in developing the world building and continuity than Mattel ever put into the canon Masters of the Universe.