Another great idea, but a little harder to implement. Adding a lot of new characters to a city map would make the game sluggish and if you have a low-end computer it would be impossible to fight in a city without constant lagging. I've already added as many as I feel comfortable with in some maps, but one or two more could be added.
But the idea of adding at least one new character to the bars is a nice one, I might do it if I ever have the time
Kepp in mind that the AI is not part of the released source code and we have very few possibilities to change it.