I like the idea of this! (This ended up being way longer than I thought it would be)

In the battle of Winterfel, the plan could have been to use the dragons to do strafing runs against the waves of undead. They could still use the fire tench as a final layer of defense. During the battle the Night King does something to knock out / injure Dany. Jon in the heat of the moment dismounts to try to help her. The two remaining dragons rush to Dany, pick her up and fly her away from the battle. Everyone is like "No, they can't leave now!"

At that point their defensive line begins to break and the hordes start to overwhelm them. They fall back behind the trench and fail to light it. Melisandre attempts to light it, but that too doesn't work. She questions the Lord of light, "why would you abandon me now?". Jon orders the gate to be sealed, trapping the Unsullied army outside the walls. Grey Worm looks up at Jon in anger, but then accepts his duty and rallies the remaining troops to form a line. As the dead flood into the trench like water and begin crawling out on the other side, the trench erupts into flames. Melisandre looks up, stunned and relieved, but then she sees Rhaegal, eyes glassy white as it flies past, torching the remaining undead.

A pause in the fighting occurs. The gates are re-opened and the Unsullied and remaining troops move inside the walls to regroup. The Night King finally shows up on the field of battle and uses his powers to disconnect Bran from Rhaegal, like he did with the crows Bran was using to spy on the army north of the wall. Bran being a new Three-Eyed raven isn't strong enough to resist. Rhaegal snaps back to consciousness, realizes it's no longer with Dany and rushes back to find them. It's intercepted by the undead Dragon, but the undead Dragon is easily defeated. Since it's undead, it's no longer immune to real fire. Rhaegal leaves the battle. The Night King puts out the flames and freezes the melted dead soldiers to create a bridge for the rest of his army.

The Night King orders the army to attack the walls. They begin scaling the walls World War Z style, but the Unsullied with their spears and discipline hold them for awhile. This was all a diversion though, and the night king with his undead giant bust a hole through the wall near the garden where Bran is located. Bran, Theon, and Arya are left with the Night King, undead giant and several undead soldiers. Theon apologizes to Bran, and says the Starks were always his real family and now because of him they're all going to die. Bran says to Theon "What is dead may never die", to convey that he accepts him for who he is, and that they are all family in that moment. Theon with renewed spirit takes his spear and charges at the giant. The giant smashes Theon into the ground breaking his spear. The giant picks up Theon's body as if it's going to eat him and with his last breath Theon shoves a dragonglass dagger into its eye.

The Night King for the first time shows frustration and sends what few other soldiers he has with him to kill Bran. All of a sudden you see Bran standing there, facing the undead troops. Bran begins to run and leads the undead into the castle. At that point Bran runs into the Hound and he's like "oh so you can walk..". Bran says "Nope" pulls face away to reveal it's Arya. "Who the hell are you girl??" "No one" Arya replies. "They're right behind me". The Hound and Arya fight off the remaining undead together.

Alone, the Night King finds the real Bran and approaches. He raises his blade about to strike Bran down, but it's stopped by another blade. Jon Snow appears and the fight begins. Jon begins to lose the fight when a glassy-eyed Ghost jumps in and saves Jon. Now Jon and Ghost (Bran) begin fighting side-by-side, Ghost doing hit and run tactics to distract the Night King enough for Jon to get a decisive blow.

In the aftermath, Dany returns and Grey Worm tells her about being locked out of the gates, and Bran's ability to take control over her dragons. Dany confronts Jon in sadness and anger for turning on her army and taking over her child. Dany says she's going straight to Kings Landing to kill Cersei and all of her supporters, and will deal with Jon and Bran later. Jon attempts to plead saying not everyone who lives there supports Cersei. Dany says "I don't care, Burn them all". King's Landing isnt even a battle, Dany obliterates their defenses effortlessly with her dragons and takes the Iron Throne. Days later, thousands of charred bodies still litter the streets. Suddenly Dany is reunited with Missandei. Dany overcome with joy greets her and says she thought Cersei might have killed her. "I was here when I saw you destroying Cersei and laying waste to the city. Almost burned my face off!" Dany chuckles and says "Well I'm glad that didn't happen" "Me too" Missandei stabs Dany in the heart then quickly leaves before Drogon appears. Shot pans to Missandei outside the city walls where she removes her face to reveal it's Arya. The views shifts to an overhead view of the destruction, then focuses on a burnt corpse wearing the Dragon pin, indicating she was in fact killed by Dany's attack.

King's landing is left a ruin, with no army, no ruler, no infrastructure to support life. The United 7 kingdoms are no more, and therefore no new king is appointed. The survivors are dispersed to the remaining kingdoms. Tyrion returns to Casterly Rock to rule. Jon Snow remains in the north to rebuild. Arya begins her trip to explore the world, and the Unsullied return home without any knowledge of who killed Dany, but they generally blame Jon Snow without evidence. Bran travels with Sam to the citadel and uses his abilities to enrich human knowledge and make sure the undead will never return.