Does it makes real sense that some of the great lords dont control cities directly? The Gardeners/Tyrells have the most populous region but even that they command loyalty of other lords, the Hightowers alone are far more powerful because of Oldtown, so i think its a world building mistake to not exist a city in the mander together with Highgarden. For the Arryns shouldnt be a city inside the vale close to the Eyrie? Greyjoys shouldnt be at Lordsport? Lannisport and sunspear makes sense to me.

I mean, yes, there should be a city on the Mander, or at least two, which is why my econ dev plan had that happen. 

However, I would caution against cities being seen as costless. A city is a large population that does not produce its own food in a world in which 90-95% of the population are needed to do agricultural labor, which requires a lot of food to be imported to it and riots if it that doesn’t happen, and which does not have a positive population growth rate, historically speaking, which means you need to continually import people as well. 

So while cities are certainly highly valuable, they are also something of a luxury commodity. (Which, fyi, is part of where GRRM’s math fails him again: cities of 500,000 are orders of magnitude bigger than almost all medieval cities.)