What is your opinion on the possibility of introducing a third tier of maps and a new faction into the game?

Yeah, and also an underwater and a celestial level. Then the asthenosphere, the planet's core, low orbit, high orbit :) If you're making a third level, don't make any statements about where it supposedly is. Because the same underground level was originally conceived by the developers as caves and, in rare cases, tunnels; it was mapmakers who started turning it into an entire "map within a map." If you're making a third level, just make it "another level of the map."