I thought it was worth a blog post.
Adventuring party as a gang, embroiled in a turf war for their neighborhood in Sigil, different gangs in charge of different doors, welcoming (shaking down, keeping the locals safe from or just flat out sending back home) anything that comes through and getting a cut from anyone who wants to venture through for economic ventures through the planes. Controlling doors allows the party to have access to different parts of the planes and through their adventures on the other side of those doors, they can change reality. Back at home, its turf wars, shakedowns and hits against rivals.
Levels 1-10 are about securing their territory and rep in Sigil and the doors they control.
Levels 11-20 are about delegating responsibilities and going through doors in order to profit from their use.
Levels 21-30 are about the players creating factions in their own image and sitting down at the table with the Lady of Pain as colleagues.
That sounds like fun.