I honestly don't know what would happen if outcast was outright removed, since I haven't played a UD character to date. But I do know what keeps me from playing there, and what would change that. I said I would get into this the next time it was brought up, and because I'm dying to get it out, I'm counting your week later thread necromancy as the next time 
Imagine an Anundor that is built next to a resource that everyone (including surfacers) wants a piece of. Brogendenstein is all in on tapping the resource and have already begun to create a wealth boon as a result of said resource, forcing Cordor and Guldorand to back individual companies on their behalf to move down there and start tapping into it themselves. Only the elves remain dubious.
As a result, the city is broken into two parts, the ever-growing population of the outer ring that prospectors from all walks of life are allowed in, and a wealthy inner city who's access is limited to the more traditional races you would expect in Anundor, already amassing wealth from the forementioned resource. A drow city* deeper in the dark has the strongest claim, but several tribes among the other races as well as a clan of duergar have all struck riches, creating the dynamic of the inner city. You could even add a human group that is easily identified by face tattoos or something to said underdark clans in the inner city, which would replace what is known as an outcast now.
What this would do is create a frontier settlement with higher stakes that everyone who likes political intrigue fraught with danger can enjoy. Harpers and zhents for example would have a city that they both want to manipulate to their own ends, but neither has a clear advantage. And you could design all sorts of things like guarding a caravan from anudor to a surface city that would be designed to challenge level 30s to complete.
Now, I didn't go too deep into the specifics because as always, my idea is just an example of how I could see it working, not a fleshed-out concept. But this is the sort of thing that would draw me to play in the underdark, because it gives my character a baseline to work with when coming up with a motivation to be there. It also serves to connect the surface and the underdark in a way that doesn't currently exist, save maybe a little on sencliff. It would also be the only city on the server that doesn't have a clear advantage toward good or evil, despite leaning evil of course. The ultimate playground for the player vs player sort, with a way for stories to foster before it just comes down to boom bash you dead I dead.
*In regard to the drow city, it doesn't have to be a settlement as we know it. Truth be told you could make the barracks outside the only accessible area, with a city wall its only visual representation in game. The point of this wouldn't be to let drow be drow off on their own in their private city, but rather let drow be drow in anundor with it making sense since they already have support for their ethical code back home, they don't need to try to bend anundor to their will to be successful at it, their way is just a piece of the proverbial machine.
Anyways, like I said I was dying to get that out of my head and into a forum post, and since I now have, I suppose the only thing left to say is "I rule!". I know it's a lot of work, and even if you think this is a good idea it would take a good deal of time to implement (though I think there are a lot of the pieces already in place, like much of Surface), so I won't be holding my breath or anything. Just food for thought for the powers that be.
