I am capping this at 4 or 5 players, myself included. I may go as low as three, it just depends. If everyone is willing to be more collaborative, we can go as high as 5, but absolutely no more.
I like Ontan's suggestion of how to break up the thread, keeping it as focused as possible. I'd also like to impose a round-robin type rule, especially in the 'main' thread, wherein no-one posts until everyone else has posted, with sensible exceptions, such as when a particular character isn't involved in the scene or a player has stated that they will be away for a time.
Another idea is that we have 'main's and 'alternates'. No one but you can play your main, but anyone can slip in and out of the role of an alternate. So, if you have an idea for a troublesome Priest that harasses you in your shop from time to time, that Priest can be played by me, but they can also be played by another player if I'm not available. This is what I mean by collaborative.
Take a look at the player's guide here
Ptolus is a major metropolis, but your general idea will still work. For example, maybe she has a shop in one of the more neglected districts, like the Warrens, where she tends to the poor and downtrodden...a place where the Church declines to visit except to make trouble for her.
Or maybe her services are directed toward one of the many ethnic groups that makes its home in the city.
The idea is that, while Ptolus is large, it is also compartmentalized, not only in terms of the various districts, but also in terms of the ethnic groups and races that live within
those districts. That way, we sort of have the best of both worlds.
You have great ideas, and this isn't a first-come, first-invite game.
Another freedom we have is that since this a standard D&D romp, we don't need to worry about the typical wizard/fighter/rogue/priest situation.
I think character creation will start with looking at the setting, reading up on the various factions, and thinking of ways you'd like to interact with them.
For example, I'm thinking of creating a halfling. I'm weighing the possibility that she is relatively young (late teens), and new to the city. Her parents were forced to immigrate for whatever reason, and have settled in Midtown. She loves
the big city. It is such a breath of fresh air from the close-confines and simple-mindedness of the village where she grew up.
Her parents, however, don't want her to forget her halfling roots. They expect her to marry a good halfling man and bear children, be a good housewife, and settle into domestic life.
Much of the drama of her life revolves around that central conflict.
So maybe that should be the start of character creation. Two lines: 1, this is who my character is. 2, this is their central conflict.
I'll get to work on a world-building thread up. Instead of rehashing the material in the player's guide, I want to focus on the changes I'd like to implement to make the city more 'street-level'.