The change of structure of the Seven Kingdoms and Free Cities at the end of the books?
With the Long Night 2.0 coming, fAegon invading Stormlands, Faith Militant taking control over King's Landing, and Ironborn raiding the Reach, Westeros is likely going to be severely pushed to its absolute limits and get fractured. The War of the Five Kings literally brought apocalypse to the continent.
As for Essos, Dany has completely destroyed slavery and economics of Slaver's Bay. The nobility of Essos is probably going to be eradicated by Daenerys. With the Old Blood of Volantis and Meereenese families destroyed, Essos is going to be torn apart with huge crisis. Therefore, the incomes and profit of the Iron Bank will drop significantly until it will go bankrupt and fall. So basically, the events in the books are a nightmare for all the powerful people in the world.
In my opinion, feudalism in Westeros will still continue but severely diminished after these events and the landscape and economics will change enormously. The Reach will stop being the breadbasket of Westeros and houses like Tyrell or Hightower will either go broke or extinct, with their lands burned and burned and armies butchered. We already know that Euron is going to destroy the Redwyne and Hightower fleet completely, raid Oldtown and perform a huge ritual that will wreak havoc. Upon receiving the news of the fall of House Tyrell and the succession crisis in the Reach, Cersei will be glad after witnessing "the fall of her enemies" and try to regain control over King's Landing. However, she will utterly fail. King's Landing will remain a place of zealotry and High Sparrow will manipulate people to storm the Red Keep to kill her and Tommen. Cersei will escape to Casterly Rock and she will learn that the golden mines had actually ran dry and Tywin had told the miners to keep it a secret. Aegon, with the support of House Martell, will be backed by the Faith Militant and sit on the Iron Throne for a short time until Daenerys lands to Westeros.
As for Essos, I can see most of the slaves killing their old "masters" and the Free Cities completely abolishing the concept of nobility and bloodlines. New banks will be founded that every person in the Free Cities could use or deposit their money as they despise and are exhausted of the old system. Overall, the concept of "the wheel" will cease to exist at the end of the books. Again, these events are a nightmare for all the nobles and slave masters and even the Iron Bank. The dominant system of wealthy and nobles will fracture.
In your opinion, what will happen to Westeros and Essos and how will the power balance change?