I don't feel very invested in the "mash-up" Cities rules we have right now. They feel very much like what they are: a way to cobble together a whole bunch of different Warhammer Fantasy units into one Age of Sigmar battletome. As an old-time Empire player, I'm really looking forward to a more unified approach to humans in AoS.
We already have ally rules for including units & characters from other armies. In my best-case scenario, they'd get their own battletomes or supplements and Dawnbringer armies can ally them in.
Alternately, it'd be cool for them to fully embrace the diversity of "Cities" and allow mixed units of "City Guard" or something, to allow people to use whatever minis they want. Like the Freeguild Guard, they could have a variety of weapon choices. Eternal Guard, Phoenix Guard, and Dreadspears could all be "City Guard with Spears," for example.
I've already got my first unit ready!