The ability to use up city improvements slots with stuff like airports, hotels and national landmarks would really make the game more realistic for example this could lead to you being able to make money from this
I do have a few ideas for the cost of these new buildings
Airport (regional) - 250,000 cash and 100 steel
Airport (international) 500,000 cash and 250 steel
Hotel - 150,000 and 50 steel
This could add to commerce or a new tourism rating which could be introduced