Of all the writers involved in these projects, Cogman has the most Thrones experience, and the most face-time with A Song of Ice and Fire creator George R.R. Martin.
There are now a vast number of historical events within Martin's novels that can be explored and Martin and the show's producers simply want to adopt some parts of it and build a new television series around it.
Bryan Cogman, who has worked on Game of Thrones since season one, is in charge of writing the proposed prequel show. He's been part of the show since the beginning... since before the beginning, actually, since he was first hired as assistant to David Benioff and D.B Weiss way before the series got on the air, before even the pilot had been filmed. Cogman has been with the show as both a writer and executive producer, being responsible for episodes such as season 7's "Stormborn".
According to Martin's latest post on his blog, the fifth script basically follows the same premise as the other four spinoff ideas in that it will essentially be a prequel instead of a sequel.
The other writers that were been previously announced to be working on the spinoffs are Max Borenstein, Jane Goldman, Brian Helgeland, and Carly Wray. "And I'll be working with him every step of the way; we're going to be co-creating the show". And that's his best guess - he stresses that no one knows for sure.
Will Theon Greyjoy be redeemed in Season 8?