In another thread I mentioned the concept of choosing a city's specialisation when it is built and have been thinking a little more on this aspect. The problem is that currently there is basically no such thing as city specialisation in the game. So if you had to choose between different city-types with unique building it would make for a much more interesting game.

The way I could see this working would be as follows:

Pioneers still found cities as they presently do.

Cities at level 1 have some common building such as workshops etc.

When a city gets to level 2, the choices available dictate the type of city is to be built:

Merchant = Hamlet -> City

Training Yard = Fort -> Castle

School = Seminary -> Mage Tower

And there could be others:

Gallows = Cemetary -> Necropolis

Tax Office = Hall -> Palace

etc

Each city type would have a unique set of buildings that would push specialisation. There would also be a few common building so you could progress even without specialisation.

Hamlets

Would grow into cities; have a higher growth and higher tax rate; have a low production rate.

Grain would be important to these settlements and these would be what funds the empire. The unique buildings for these cities would focus on growth and tax income.

Forts

Would grow into castles; have a slightly lower growth; low tax rate; but high production.

Production and unit bonuses would be the important resource and these settlements would build the troops for the empire. The unique buildings would focus on units and production.

Seminaries

Would grow into mage towers; Have very limited growth; no tax income; and low production.

These would be specialist settlements that focus on magic, research and special units.

This would mean less choice when adding buildings to your cities - but much more important strategic choices when you level up.