This might be a very odd take, but, has it ever been considered to have more than one single settlement in the underdark?
The reason why I am asking is that surface characters that, say, fall out with one of the main cities generally have other cities to join. Andunor is the only available turf for pretty much all underdark races, and has led to a very odd situation, where UD characters have to play extremely nice to each other in order to not be eradicated from the only settlement they have.
On one hand, two options (perhaps a "moderate" melting pot in Andunor and a more classic drow settlement?) might split the playerbase and thin out the UD playerbase. On the other, it might actually invite more people to play in the underdark, as it would create all sorts of dynamics and grounds for UD vs UD conflict. Right now, houses and factions that are not welcome in Andunor don't even have an option to live in the wilderness, and as a result often just fade away. What are your thoughts on it?