A few things here. Again, bullet points will serve me fine.
- Factions: I actually like post faction war Planescape better. The factions seem to dominate RPing choices when they're hugely powerful. With that in mind, the booting of the factions is a good thing. You can still have a faction asociation and powers without creating a situation where it's the only possibility. I also love the ideological conflicts between the outsider factions. My own bias influences this decision, obviously.
- Sigil's size and population: REALLY BIG!!!! I felt Sigil was always too small. Make it really big and really dense. Like small modern country big at least. Maybe bigger. Crowding should be an issue as always
- Maybe tke the dabus out of building construction at least in part? That raises the question of who owns the land in Sigil. A dialouge on this would probably be good. It's important
- Portals and large scale transport: Trains? maybe but it hardly seems safe. This is a major issue in Sigil--you have to get food and water in and finished goods out.
- Pollution. The lady would probably take issue if things got too bad, but it can't be a good situation
- Public transport: Cars would be a problem given congestion, but so would trains (tunnel=good place for a portal) Maybe roads reserved for busses and specially liscence vehicles. Ideas here are good too.
- Economy: Industry would be big in Sigil, maybe also technology, but above all, I see it as a financial and trade capiitol of the multiverse--it's connected to everywhere else.
Size: Well think of Sigil as some thing the size of the LA metroplex here in the US or something the size of London. Something that big does not have huge as in some places it is the size of some counties.
Land: I see three things about this I paid my money for this land at its mine, that may or may not work in some areas. Land that has been part of my family line for centuries, which my or my not work. Or the ara where might makes right.....
Portals and trains: Something along the way Eberron does it for trains.
Pollution: Some areas area worse off than others and some are real hell holes.
Transportation: anything and everything.
Economy: I agree with what you see on this.