@bedir than
My central conceit is a (human) society, assailed on the surface of the world, had its most powerful magic users perform a feat of unimaginable sorcery and moved the population to the
interior of the planet. Now, generations later, they have branched out from the initial settlements and discovered that, somehow, they are not alone.
My intention is to have the setting remain as generally compatible with everything else as possible. After all, most adventures don't really concern themselves with what's actually up in the heavens; as long as you can tell whether it is day or night, who cares what's beyond the sun? I've been running a game set in it for a few months now, using a modified version of the D&D
Rules Cyclopedia, and none of my players have even noticed the abnormality (aside from a casual remark about the lack of dwarves). This experience tells me the idea works.
I just thought I'd reach out beyond my bubble a bit and ask for thoughts. I'd ask on Twitter, but frankly the reach and likely response rate here is much higher (also, the greater degree of nuance available on a forum as opposed to 140 or whatever character tweets).