I would have written that there must always be a Night King because of the threat that Bran imposes on The Night King.

The Night King or atleast his magic cannot be destroyed. So towards the end of it, Bran takes up the mantle of The Night King and travels north with the army of the dead.

I would write that Jon defeats the the Night King in single combat but they realise that the army of the dead is running rampant, mindless and uncontrollable. This is where Bran submits himself to the magic. Bran tells Jon how the magic was created, and Jon puts obsidian in his chest. He then goes North and sits in the North, keeping the undead away from south.

This allows the Nights Watch remain in the realm. Jon becomes King Aegon Targeryen with Queen Daenerys Targeryen ruling as co-monarchy. Jon would doctrine that the Wall to be restored, to its full glory. And i would write that Jaime kills Queen Cersei. Jaime becomes the Lord Commander of the Nights Watch but its no longer an exiled job but an honorable post full of knights, warriors and honorable men. No longer cuthroughts, rapists and thieves. All 18 castles are to be built, with the seat of its power being Castle Black establishing House Black with a representative on the small council. 

Tyrion would inherit Casterly Rock but serve as hand to the King and Queen. 

Sansa would remain as Queen of the North and serve as  an allied country to the six Kingdoms and the Wall. 

Because of the potential threat that Bran could be potentially going rogue, the Nights Watch is at full force with thousands of men manning the wall with the full support of Sansa and her banners aswell as the six Kingdoms.

All houses would be required by doctrine of the King and Queen to supply men to the wall High Lord and lowborn but that they could still marry and inherit titles. 

In terms of succession, Lord Gendry Baratheon would inherit the Iron Throne as Daenerys cannot provide an heir for Jon. 

I would have written 'There must always be a Night King'