So, for me, part of building a world for an RPG is knowing how the world worked before the players came along, and how it'll work after they go. Yes, some places will be greatly affected by the presence of the PCs (current fantasy homebrew has several "Mixed-Race" settlements, humans are a minority - several of the distinctly non-human settlements will treat any newcomers with suspicion and interest), but I hate the idea that, no matter how large the city you want to portray is, it'll always come down to existing when the players need it.
One of the main cities in the Human/Halfling settlement will be a classic Hive Of Scum and Villainy - the kinda place where the Big Bad has a strong influence, the bars are dirty, the magic black, and a trip down the right back alley can net you anything. This seems to me to be the definition of a bustling metropolis, and I'd hate to see it stagnate and bore my players, or worse for somewhere I pictured to be so vibrant and chaotic to end up revolving around their actions.
I want mystery, intrigue, politics and more, all for the players to dip into if they wish - but I want it to be about the city, not them.
In an effort to make this happen naturally, I plan on wholesale lifting the Relationship mechanic from West End Game's Smallville, except for the dice-ing part (that might be silly). However, by having a chart of who interacts with who, how, why, etc. I can make the place seem way more... alive, I suppose.
So, yeah - I will post up the first draft of the map when it gets worked up!