While the Thrawn trilogy was really good stuff, there's far too much stuff going on to compress down to ~6 hours of film, so you'd either lose a bunch of the subplots, or they'd get mashed in very awkwardly. On the plus side, Benedict Cumberbatch would be pretty much an auto-include as Thrawn. On the minus side, that would have everybody comparing it to Into Darkness and not actually looking at whatever was actually done.