I don't particularly like the slot mechanic in cities as seen in 1z.  It drastically limits your options on how you build up, as every city must be specialized due to size constraints rather than economic ones.

My proposed alternative: Decouple city level from available slots.  Each house directly contributes to new slots, while not requiring any slots by itself.  Houses require food instead.  City levels are based off how many houses are present, and represent how many houses are required per new slot - a level 1 city requires one house per slot, a level 2 requires two, etc.  Make the city centers directly upgradable, with each step up increasing the cutoff points for each city level - first tier city centers can only support 10 houses before the city reaches level 2, second tier city centers can support 20, etc.  Note that all numbers are just examples.

This method keeps the requirement of infrastructure to support your industry, but at the same time is much more flexible in making use of available resources and has no hard caps on cities if you with to spend the time and money to build them up.