In Beta 3 we've had the interesting ability to specialise cities by choosing a % bonus to research, arcane knowledge, gildar or materials production with every level the city gains.
Furthering this idea, why not link particular city improvements with such levels of specialisation?
E.g. Hosten's library is only buildable in a city which has a 30% reasearch bonus. Or a royal mint is only buildable in a city with 30% gildar production bonus. perhaps the great mill could be similar for materials production?
Having such buldings would make cities look more specialised, something that they really don't still (except for whatever resources are near them).
Depending on the degree to which stardock wants to encourage specialised cites -and to avoid city spamming, perhaps this principle could be extended to more common buildings like libraries, academis, universites, markets, tax offices etc?
At the very top of the city and tech level, perhaps there could be buildings analogous to Civ's "small wonders" -i guess like Hosten's library or the royal mint, which really focus a cities output on a particular sector/resource and of which there can only be 1 per nation? This would also like gal civ's "manufacturing capital" etc.