I'll be so excited if this allows multiplayer city building. Imagine the possibilities...

First of all, you get to collaborate and build things faster. You could have a voting mechanic where the citizens (AI) resolves conflicts. You could assign roles to subordinate players to manage things like traffic, services, new developments, etc. You could implement trade across city lines, and have acquisitions. It could easily turn into a MMO of epic proportions; with whole worlds built out.

So. Many. Things!