This project is likely not going anywhere anytime soon. David Benioff and and D.B. Weiss have been selected by Disney to write and produce a new series of Star Wars films following the conclusion of Game of Thrones in 2019. This will likely take up all of their time, leaving the project dead in the water if not permanently abandoned.
If not maybe some other Civil War AH like say Harry Turtledove's Southern Victory series could be adapted by HBO instead.