"Skooma guild" does sound ridiculous. But I think the ability to join one or two "less than legitimate" organizations would be cool. In Morrowind, all of the organizations you could join were, at least somewhat, legitimate. Even the thieves guild and Morag Tong (sp) were considered legitimate (if not always respectable) organizations.
In Oblivion, the thieves guild and the dark brotherhood were both underground, which I liked, but neither seemed all that gritty or nefarious. Neither of them had much of a purpose, no central focus. Giving a faction like this a large goal (like controlling all of the drug/slave trafficking in the area) would make them seem more important in the overall story.
I like how this is put. I would also like to add to it, by saying I liked how certain clubs and areas in both games (mainly Morrowind, but a bit in Oblivion, too) were their territory. I'd like if they expanded on that, with gang areas in which you could get your ass beat if you messed with the wrong person, or if you hung around enough for people to trust you it would be easier to join these illegal guilds. (On top of that, it should be harder to join. There's a reason illegal orginizations stay under ground, and that is to not be found. Trying to join goes directly against that, so why don't they try harder in hiding?
There should still be recruiter NPCs, of course. It should just be a lot harder to get them to trust you. Maybe it will depend on how often you hang around, or maybe it will depend on how well you preform small tasks for them. Like, you need to do a few quests for them that aren't stated to be part of the guild, but still relate to it. They could be seemingly legal at first, quests to go deliver this gift to a good friend of mine, it's her birthday and I wanted to get her something (but the package has something like drugs in it if opened). Then they could become a little more obvious, until you're trusted enough to do it.
I like how Dark Brotherhood recruitment was in Oblivion. You had to murder someone before they considered you. It would be better if someone like the Grey Prince didn't count, though. It would have been better if after you became the Listener you were actually approached by the remaining branches, though, and there was actually some sense of leading them.