The battle can be a short and easy one due to how OP Halvard and some magic can be in the right way, like just mounting a airship and flying North and destroy the place with magic cannons Or I can wait and have a battle on the wall.
I am leaning to the short, because once Greyjoy rebellion happens the North are gong full colonizer mode and getting as many colonies as possible.
Next while the North makes colonies we get court wizards in every Noble house in the North to get them to stay loyal to the Starks/me. So when Robert dies we declare independence and tell everyone south of the neck to fuck off.
And since the white walker will be dead we the wall began to melt so we can expand North and "subjugation" campaign in the true north.
As the south tears itself apart we will be making bank off trade with Dorne since they are already basically independent and try to get an alliance with each other.
By then the first graduating class of the School will be out and they can work on expanding the North which will need a new name as we are no longer just North. They will head to Dorne and put in place the loyalty enforcement ward like in the North.
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During this I have an idea that we summon Lilly Potter and her Daughter on Halloween, then during the Triwizard tournament Harry gets summoned then we have to deal with the wizarding world their. Just an idea IDK if I will do it.
Another side story will be The Chosen group of kids meeting and making an adventure party and starting up adventurer guilds, then them being hired to map the area we are colonizing.
Governmental changes that will turn the North to a parliamentarian government leaning closer to constitutional Monarchy.
Leave ideas about the future.