The Underdark currently contains one settlement with three districts and a small Svirfneblin outpost. However, we have advanced far past the days when the Underdark had only a few players to its name. In this day and age, real estate below is hard won. And the hub is at most times busy or soon to be busy. However, this expansion of players without expansion of settlement options has quickly turned into problems.
Andunor is easily seen as only one settlement by those above, below technically there are three districts. But due to close proximity, these operate far more often like factions within the city all vying for control rather than feeling like separate governments. In addition, the number of quarters and shops reflects closely to Cordor, thus these three settlements are actually three settlements in a trench coat to make one city.
Why not Sibayad you may ask? A vast majority of UD races do not have that option due to lack of citizenship. Drow(~ 600 monthly active), Orogs(~ 150+ monthly), Duergar(~ 100 monthly), Kobolds(~ 100 monthly), Goblins(~ 80-100 monthly), Hobgoblins(~ 50 monthly), Ogres(~ 40 monthly), Bugbears(20 monthly), Troglodytes(~ 20 monthly). Around 1170 characters per month with only one choice for settlement, because as discussed above, Andunor is more like one settlement rather than three for many reasons. Andunor of course is home to many more that those, but they were not included as they could move to another settlement.
Okay okay, but UD population is low, you don't need another city. Andunor counting all three districts has currently 530 citizens. The second largest count of citizens? Cordor with…357. Guldorand, 313. So, Andunor has a citizenry of heading towards the two largest surface settlements, all while only having the amount of shops and quarters of one, or if you count Treadstone more quarters but many are very small rooms in a higher portion than most surface settlements.
Proof of numbers on quarters and shops.
Around Hub/Sporefarms/Shipyard
Guild Quarters:2(7 total quarters across guild houses)
Quarters:14
Shops:16
Sharps
Guild Quarters:2 (4 total quarters across guild house)
Quarters:14
Temple: 1
Shops:8
Citizens:141
Devil's Table
Guild Quarters:
Quarters:14
Temple: 1
Shops:8
Citizens:166
Greyport
Guild Quarters:2(7 total quarters across guild house)
Quarters:12
Temple: 1
Shops:15
Citizens: 223
Treadstone:
Quarters:27
Shops:6
Andunor Total:
Guild Quarters: 6 (18 total quarters across guild house)
Quarters: 54(81 with treadstone)
Temple: 3
Shops:47(53 with treadstone)
Citizens:530
Cordor
Guild Quarters:6(19)
Quarters:47
Temple: 2
Shops: 57
Citizens:357
Guldorand Citizens: 313
Having put all my evidence forth of why I feel the diverse and clearly populated UD deserves to be looked at for a settlement, there is also the factor of giving RP diversity via options. The districts of Andunor give a little variation in choice, but you are still an Andunorian, it is much more minimal to the difference between Cordor, Guldorand, Bendir, Myon, Brog, and so on. And this is on top of the vast variety of races and cultures to be just stuffed all into Andunor. Adding even a small settlement, would give more choice to the RP of underdarkers, conflict between their cities, possibly even interesting differences between the settlements interacting with other settlements and groups like Sencliff.
My own personal suggestion, due to numbers and their strong culture is a majority drow city. Around 600 drow a month is the highest in race numbers. In comparison, Elves have around 1000 a month if you add all the types including half-elves, and more like 800 or a bit less total if you don't include them. And that racial population is enough to justify Myon. In addition, drow are known for their strong culture, many seek to play them specifically for that. Aggressive Lolthitism can also be very restrictive to much other RP, as the religion seeks to make all others conform to their culture. And this works against the Skullport style city Andunor seems to be sought to be. Over the past year plus, it has been frequent for fights between drow houses and other city factions or between houses to take place. Which is frankly, bad for business in a trade city. And my suggestion isn't that their RP is wrong, simply, the UD needs more space to spread out for varied RP. And it seems like a city focused upon drow house rp with their many retainers and slaves along, would potentially make a nicely sized city to allow that RP, while allowing Andunor to be more Skullport trade city in style.
That's just my idea and take though, looking forward to hearing other settlement ideas, disagreements, and everything else!