If i were them i would state that factions that are eligible for a free city ie for Living City: Sylvaneth, Wanderers, Freeguild and stormcast, you would swap out their faction keyword for the "Free Cities" Keyword. It makes sense in the aspect that these guys fight as a cohesive whole but removes some of the complications of things not working correctly when it comes to abilities. Or they'll make it a supplementary keyword to be added when the unit is added to a list like the chambers or groves etc. Either way it would allow the factions to play as individuals or as the "free city" narrative they're pushing.
If they take either of these options it also leaves open the chance to revisit the individual factions if they fancy expanding them *crosses fingers*