HBO isn't about to put all of its dragon eggs in one basket. The network is reportedly set to order a pilot of second Game of Thrones spin-off, and in a way it could serve as the perfect compliment to the GOT prequel pilot that's already in production.
According to Deadline, the possible series would be adapted from George R.R. Martin's companion book series Fire & Blood (a second volume has been announced but—in true Martin style—has not yet been released.) The book details the history of House Targaryen in Westeros. It begins with Aegon I Targaryen's conquest of the Seven Kingdoms, covers a Targaryen civil war, and creeps into the post-Dragon era of the monarchy. It's unclear whether the series would focus on one particular time period or be more of an anthology.
In contrast, the other series, which remains officially untitled but has been referred to as both "The Long Night" and "Bloodmoon," reportedly focuses on Westeros before the Targaryens arrived. It features an absolutely loaded cast headed up by Naomi Watts, and though a pilot was shot over the summer, HBO still hasn't decided whether or not to green-light the project. If both series do move forward, it's quite possible they could live comfortably in tandem. From what we know at this point—which, admittedly, isn't much—they do seem quite different. Here, how the two series (which we'll call Bloodmoon and Fire & Blood for now) stack up.
Bloodmoon: Unclear, but as we've previously concluded it remains sort of unknown how common dragons were in Westeros before the Targaryens brought them over to conquer the continent. There are some references to an occasional dragon in the land before all of that in Martin's work, but they're few and far between.
Fire & Blood: We would assume there would be lots and lots of dragons considering how interconnected the beasts are with Targaryen history.
Bloodmoon: Take it from Martin himself who told Entertainment Weekly of the series, "There are no Targaryens."
Fire & Blood: Yes, duh.
And Those Lannisters?
Bloodmoon: Again, Martin has said that the Lannisters aren't on the scene yet when Bloodmoon begins. In fact, their eventual home, Casterly Rock, is still owned by the actual Casterlys. The series could, however, eventually introduce the family and show how they tricked the Casterlys out of their castle.
Fire & Blood: Quite possibly. By the time the Targaryens show up in Westeros, the Lannisters are a powerful family. They appear quite frequently in Martin's Fire & Blood book.
Ok, But What About The Starks?
Bloodmoon: Martin has confirmed that Starks are very much around during the prequel.
Fire & Blood: The Starks pop up now-and-again during various parts of Targaryen history, but often they're just allowed to chill up north.
Bloodmoon: Yes, Martin has confirmed that the White Walkers are active during the time of this prequel, and there are enough clues to suggest their first defeat may be a part of the story.
Fire & Blood: Probably not. The Targaryens never had to deal with the White Walkers during their entire long reign (until of course, Game of Thrones) to the point that many people had begun to assume that they never even existed in the first place.
So, in short, while neither series alone gives us everything we want out of more Game of Thrones television, together they could deliver the goods. In fact, it might not be too silly to think of one as "the one with White Walkers" and the other as "the one with dragons." Whether either, both, or neither actually makes it to the airwaves, however, remains to be seen.