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

The underground level is simply caves. Sometimes tunnels.
In The Restoration of Erathia, there was only a network of tunnels in a couple of missions. And that's it. And even then, it was based on the plot.
Nigon is an island with a mountain, from which a network of natural and dug tunnels and caves extends.