Note: Though this quest heavily involves the Thieves Guild, the player does not have to be a member of it to do this quest.
On coming to Andothren, the player is asked to help out with a minor problem: something like getting a debt payment from an NPC, confronting a pick-pocket, delivering a package across the town, etc. This minor task puts them in contact unknowingly with a member of the Camonna Tong. It was in effect a casing of the player for some later shenanigans, which go like this:
The local authorities have heard that someone is trying to ship a batch of slaves up the Thirr River. The issue is not the slave smuggling persay, but the fact they are all Argonians and House Dres is trying to claim they are the only ones with the proper ancestral authority to sell these particular slaves. (Some complicated bureaucractic/political thing.)
The player is asked to help guard a ship's cargo going up and down the Thirr River. This may be suitable as a mini-questline for House Hlaalu or the Imperial Navy (aka Imperial Legion) or the EEC or the FIghter's Guild. I'd most suggest Fighter's Guild as otherwise it's hard to make interesting quests for that faction.
I propose a quest/questline based on the life of Christina of Markyate as recorded in her hagiography. Here is a bit of information about her without having to read the book: https://en.wikipedia.org/wiki/Christina_of_Markyate.
