My thought was to have him start as a rat catcher with bonussed stats (to beef him up a little, as he's going solo) recently employed by Wahlund's Rat Catchers (from "A guide to Übersreik").
I want to start off with a railroaded, actionpacked segment where the main character is taken with the "inner core" of Wahlund's Rat Catchers down to hunt the "bigger rats" (Skaven), in search of a lost Sewer Jack. Wahlund thinks the Skaven took him, but needs more muscle to attack their lair. Therefore our main character is chosen for "promotion", from hunting normal rats to Skaven.
The rat catchers find the inner skaven lair empty (after having bloodily fought their way in).
Empty handed they return to the surface.
Our main character lingers in the sewers (If you have a good reason, please let me know. I haven't come up with one yet), and meets the missing sewer jack! HOWEVER... it's really the doppelganger Brandt (who has already devoured the real sewer jack), and a dangerous stand off occurs... With Brandt trick our main character into letting his guard down, and then kill him?
Well... that what I have so far.
After this adventure, I will open up the world and make it more sandboxy.
I will be grateful to know your thoughts and additional ideas
