Well, I mean for the characters to drive the plot for the most part. Towards that end, I am going to be a little picky about what characters I accept into the cast. I have no limit to the number of characters in mind, but I do want to have a wide variety amongst them. When characters start being posted, I'm going to start PMing between peeps to try to make little connections. 

I want a sort of central hub for all of the characters. Something that they have in common, but not necessarily something they're always doing or being at. I was thinking a cafe, bar maybe even bookshop that they're all patrons of. 

People can go ahead and give me the sort of character they'd have in mind, so that we can start bouncing connections around.

To keep the plot moving, I was thinking of having periodic "events" that everyone, or some of the people, can participate in. I also mean for this to be an RP where people can join at pretty much any time, or that they can leave and come back(with character excuses) without too much consequence. Or with heavy consequences, if that makes for a better story. 

There may be some action and adventure on occasion, but the story will be primarily interpersonal.