Been thinking about the chances of something like this, something which can cover part of the period leading up to the current sequel trilogy and the First Order's rise, give us an idea of who Snoke is, Ben's full fall to the dark side, more chance to develop Hux and Phasma's roles leading up to Episode VII and VIII... but the problem now is that it sounds like Mark Hamill, the guy you would definitely need to make this movie work, even more than Harrison or the late Carrie Fisher, sounds like he wouldn't be keen on coming back. So would that kibosh a film like that ever getting made to reveal the stuff going on in that period?
I figure that Rian Johnson will be doing his trilogy as a Legacy style piece with a good generation or two after Rey's time with the newest force users, and the Game of Thrones guys trilogy will almost certainly be an Old Republic tale set some good time before Phantom Menace, so it would be down to someone else to direct it, but I'd like to see it happen.