Unlike any other kingdom, North's economy prospered in war due to heavy compensation other kingdoms had to pay for starting the war and for release of hostages. Starks instead of hoarding the gold, invested it in the North. Helping in renovating the Keeps of its lords, helping in building towns for every major lord and financing trade. Starks even got their Valyrian Steel sword, Ice from one such war, where they were able to take King of the Rock, Lord Lannister hostage and demanded a valyrian steel greatsword of the best quality possible.
They also started making use of the unadministered land like Sea Dragon Point, Moat Cailin, North Reach (New Gift) and Cape Kraken, which Flint's can't properly administer due to ironborn, they made the Sea Dragon Point into the western naval headquarters of the northern fleet and is administered by someone from a noble family and keeps changing every generation. Moat Cailin was turned into a military stronghold, similar to Night's Watch but only willing people can join it, that too from North only and will receive a payment for it.
North Reach did not have any lord at present and is administered by second or third sons of noble houses in return for payment like a steward, till a lord is selected for it. As for Cape Kraken, it was difficult to properly manage it with the constant ironborn attack, so a fourth of its area in the east was given to House Reed, who ruled the Neck and the Crannogmen, for their contribution in the many wars of Faith, another fourth in the southern part was given to a second son of King Stark to form a branch house there, leaving only half of the land for Flint's which they already administered. It took a lot of burden off of House Flint of Flint's Finger and helped them in investing the gold in the land which was going waste in trying to protect these parts, leading to their prosperity.
It proved a wise decision because in the next few centuries, all 3 houses discovered some berries and grapes that were native to the region as The Neck and Cape Kraken is the only region of the North where it rains properly. They turned this into their strength and dived into the business of wine making. The wine made was very nice and sweet and they made two version of it, one that is strong as any normal wine and second that is slightly less intoxicating meant for women. They were instant hit in the market and generated a lot of revenue for the 3 houses.
Starks would go onto conquer the island of Skagos and its neighboring islands, but Skagos along with Boltons will rise in rebellion against Starks a couple of centuries later. After suppressing, the Boltons and leaving only the heir, who was only 6-year-old at that time alive, they went on a naval invasion of Skagos. It would turn out to be a costly affair due to unfamiliar and harsh terrain, but Starks will come out victorious and will wipe out all 3 noble houses of Skagos. The daughters of these houses will be married to second and third sons of House Umber, Hornwood and Woolfields and will make a new branch house of Starks who will take the name Skag after the island was raised who will be the overlords of all the Houses in the island of Skagos and its neighboring islands.
New Houses will also be raised out of those who performed well in the war on the neighboring islands. People from the mountain clans were also encouraged to migrate to Skagos as the island of Skagos is mountainous and they were very experienced in living in mountains. It was also discovered after a few years that the valleys of Skagos are very fertile along with the neighboring islands. The fertile valleys and granite found in the Skagos along with the rearing of unicorns and one-horned goats. The milk of these goats is very delicious and could be turned into very delicious alcohol. Many of these goats were given to mountain clan chiefs to pacify them over the loss of people.
Also, all the mountain clans were united under House Burley as the Major House who will rule over them. They may not hold the biggest land out of mountain clans, but they held the most fertile and populous lands. Burleys had a new stronger and more defensible fort of stone built to show their status as a Major House of the North.
Similarly, many more keeps were built and renovated, the Ryswell Hall which got very damaged in an ironborn invasion had to be completely rebuilt and was named Blood Stable. Dustins have their fort expanded and made completely out of stone, called Barrow Castle. Karfort and Last Hearth was also made much stronger and completely out of stone. Mudds have a strong castle called MudStone built along with a well-fortified city in the Stony Shore. Castle Hornwood was also upgraded along with keeps near the sea like Ramsgate, Widows Watch, Oldcastle etc. Reeds with the increase in income through wine trade also rebuilt Greywater Watch out of Ironwood.
Starks also discovered that lands near the river were more fertile than other lands and so they heavily invested in developing farms near river and proper irrigation. This led to an increase in food production of the North and reduced their food imports by thirty percent. It is a good thing that Riverlands is so divided because whenever Reach starts a 'Holy war' with North some of the Riverland Houses are always there to provide North with food. It is also the reason why Starks never interfered with Riverlands because a united Riverlands may refuse to sell the food they need and importing from Essos is dangerous due to high number of slaver ships roaming in the narrow sea.
Over the years trade with the free cities increased and they started building friendly ties with Valyrians as well. Ofcourse they were still against slavery, but business is business. Taking inspiration from Valyrian Dragon roads, Starks also started building proper roads in their kingdoms that connects all major keeps to Winterfell and each other. Relationship of Starks were so great that they were able to eventually befriend House Belaerys, one of the strongest of the 40 dragon riding houses of Valyria.
As for the South, Riverlands is busy in their own conflict as always with a new king popping up every millennium or fighting with Stormlands over what will one day be known as Crowlands. Vale is busy with their conflict with mountain clans. Reach is busy in fighting two-way war with Westerland and Stormland who both want a piece of fertile Reach for themself.
Dorne is the most interesting of them all, while it is also busy in their fight for Kingship but when it looked like a House will finally become the King in next few centuries, suddenly a new power emerged and became King or should I say Prince. It was like this Dayne's who over the years were able to take over all the lands around the Torrentine river and the west coast of Dorne. They were rising in power steadily through the fertile lands near Torrentine and trade on the west coast.
They were also able to capture the Arbor Island when the Redwyne fleet was almost completely destroyed in an ironborn attack. They wiped out the whole House Redwyne so that they can't shift back to Reach. Reach whose navy was almost completely decimated and coast was facing constant attack from ironborn couldn't do anything but grind their teeth. A few decades later when again Reach was busy in a two-way war with Westerland and Stormland they marched North in the Dornish marches and were able to capture the lands of House Costaynes and Cuys and used the Starks way and married their daughters to their second and third son who took their names.
By the time Reach was in any condition to retaliate, it was too late as the Dayne were deeply entrenched with in the lands. So again, they would have to let go of these lands or pay with lives of tens of thousands for the lands. With the increase in number of men they could call because of the fertile and populous lands of Reach they were able to expand into the central and southern Dorne. But as they were biding their time after the recent expansion of lands a new power emerged, the Rhoynar.
After getting defeated at the hands of the Valyrians, before they could come to capture their lands and turn them into slaves many of the Rhoynish ran away, some to basilisk isles, some to summer isles and stepstones and the biggest refuge contingent led by Princess Nymeria of Ny Sar reached the coast of Dorne, in the lands of Martells to be exact. Nymeria whose ships while filled with only children and women (most of the old people stayed so that more younger ones could run away, and the men died in the war with valyrians) but were still an increase in the population of Martell lands. Many of these women were also warrior women of Rhoyne, as they believed in gender equality, or were trained by Nymeria over the years to fight against slavers.
Mors Martell, the lord of that time was very ambitious. He agreed to grant them refuge if Nymeria would help him become the King (Prince) of Dorne. Rhoynars were better in working steel than Andals, they were trained fighters thinking of this Nymeria agreed and after a few years of preparation, in which Nymeria and Mors married and many Rhoynish women married with dornish, started a series of war. Most of the kings and lords were easily defeated as the rhoynish water mages could cut of the supply of water from any keep, leaving them to die out of thirst and a siege which was mostly the reason for survival of rival houses, as they couldn't grow much food to feed large armies.
So, only the option of frontal war was left and Nymeria proved far better in it than the lords and kings. The water mages also proved themself here as well as they would just drown the armies by turning small streams into river when they were crossing and what not. The only big threat in their way were Dayne and Yronwoods. But the Daynes chose to surrender as their castle, Starfall and capital city was located on the mouth of a river as is most of their land. He didn't want Nymeria to flood his lands as that will lead to countless death and the end of his House. The newly turned Lord Dayne married the High priestess and strongest water mage, Druselka. With the might of Dayne merged with Martells, they were easily able to defeat Yronwoods but Mors Martell died in the battle leaving his daughters as heir. While Daynes may have given up their crown but the lands where the once ruled were still more loyal to them than Martells and their strategic marriage with Druselka ensured that they also have Nymeria's strongest weapon with them in future. But he didn't know that the weapon will soon become redundant for the next few centuries.
Nymeria took the hold of Dorne as Princess according to rhoynish customs as they don't have kings. She suppressed many rebellions and fought with Storm kings and Gardener Kings. Her daughters took the command of army after she grew too old to fight. She passed away in her sleep after ruling for twenty-seven years and her eldest daughter took her place as the princess of Dorne. She left a strong kingdom and a home for her people who helped in improving the lifestyle of dornish drastically but over the years the power of rhoynish water mages began to weaken with the distance from the river Rhoyne. They, who were able to command rivers once were barely able to lakes and most of them could only command ponds.
Author's note:
Skagos is at the same latitude as the Gift and New Gift, so it faces the same geographical conditions and Skagos mountainous terrain should help in stopping clouds travelling from Shivering Sea and Narrow Sea for rain in summer.
As for Reach, I always thought it was way too big for some lands made completely out of open fields and plains.