2. The War and the Pact
What it is: As humans tend to do, the First Men did not like the idea of living in the middle of a dense forest without proper houses and villages, so they started to fell trees, including the weirwood trees sacred to the Children of the Forest. Before you knew it, an interspecies war broke out.
That might not have been the whole story. Maybe there were too many humans too soon, and both races had to compete for resources. But either way, war ensued, the First Men fighting with bronze weapons and brute force and the Children with magic. The Children shattered the Arm of Dorne and subsequently flooded the Neck using something called the “Hammer of the Waters,” probably some kind of spell. The war ended with the Pact: the Children were granted the deep woods and the First Men got to run the rest of Westeros. The First Men agreed to no longer cut down any weirwood trees, and subsequently became followers of the Old Gods, their interpretation of the Children’s bond with the weirwoods.
Why it could be awesome: This is a proper war with strong fantastic elements, which has been what Game of Thrones does best over the last couple of seasons. We would also get a much deeper understanding of the Old Gods, or rather of the mysterious figures and events the First Men came to call by that name. And the Pact is one of the most important watershed moments in the history of Westeros.
Why it could be terrible: This could mean a lot of CGI warfare, which could look absurd on a larger scale. Another issue is that HBO has very little to base a show on. This conflict surely had leaders, but we don’t know any of their names or how and when and where they fought. It’d be a tall order of a show.
How likely we are to get it: 90% (Episode 1? Season 1? Only a flashback?)