One of the consistent favorite topics on Mark Rosewater's Tumblr blog, Blogatog, is what sorts of planes Magic blocks might be set on in the near and far future. Recently he was asked intriguing questions about two different settings both having to do with enclosed, cramped spaces.
When cairnwanderer punnily asked, "Is it possible we'll ever see a 'Subterranean' plane/block, or do you find the idea too UNDERwhelming?" Rosewater responded:
It’s an idea that keeps coming up which I’m sure means we’ll eventually do it one day.
Of course, one of the immediately apparent issues with a subterranean block is that flying - the most-used keyword in the game, which only failed to appear in Fallen Empires and Portal: Three Kingdoms - would be hard to justify flavorfully. Perhaps, though, R&D could make a gimmick of a "no-flying" block and provide many alternate forms of evasion. While some responses to this were jokey - modernrecluse's "MARO CONFIRMS HUMAN MOLES AS A CREATURE TYPE!" and tijmenndg's "Underground sea says whatsup *shakes fist at restricted list*" - others had serious suggestions. User trolledthrun wrote "Subterranean block with journey to the surface in the third set. SURPRISE!!! We’ve actually been deep inside Dominaria!!!" while doombladez said "Flying could happen, giant subterranean caverns are a possibility."
Meanwhile, rageofthestoneclan had a slightly different suggestion, asking "Is a prison plane out of the question," to which Rosewater said:
No, it’s on the short list.
There were a handful of enthusiastic audience responses to this, including one comment about the possibility of parallels between white and black-aligned characters, with white villains and black heroes like in Kamigawa block.