Islamabad is the capital of Pakistan. The city as such only exists for about 50 years, for 30 years it is the capital of Pakistan. Islamabad is a planned city. It was designed on a drawing board. The building took 20 years. It would have been a lot quicker but Pakistan kept running out of money.
Islamabad might be the official capital but it isn't the biggest city in Pakistan. Karachi is 15 times the size of Islamabad. The Pakistani capital is growing rapidly though. Right now there are 6 other cities bigger than the capital but this might change soon.
There are 200 million people living in the country and the rising oceans pose a serious risk for the lower parts of the country. The migration pressure will probably rise as much as the water does. The capital Islamabad will most likely be one of the more popular destinations.