HBO is working hard behind-the-scenes to keep that sweet, sweet Game of Thrones train running well after David Benioff and D.B. Weiss' main series comes to an end. We're currently up to five spin-offs in development, but the only one with concrete details and a timeline is the still-untitled prequel pilot written by showrunner Jane Goldman (Kingsman: The Secret Service) and directed by SJ Clarkson (Jessica Jones) with Naomi Watts leading the cast. The latest, according to HBO Programming President Casey Bloys [via EW], is that the pilot will begin production in early summer …