A nice parallel subplot could center on Vader
I'd go even further: I'd love to see another clash between Vader and Ben, before they get old. And the reason for this - beside a possible kick-ass Force battle between them - is the fact that since ROTS I have a problem with two quotes from ANH and ROTJ: "You can't win, Darth" and "Obi Wan once thought as you do." In my opinion, there is a huge emotional gap between "You were my brother, Anakin" and that cold "Darth" quote. Something must have happened. Something more dramatic than 20 years of solitude. And the other quote suggests that at some point Obi Wan actually tried to bring Vader back to the light side, which never happened in ROTS. Who knows, maybe there's enough room for more than one movie about it. McGregor is there. McDiarmid too. Heck, even Christensen (if we really need to see Vader without helmet, to which most of you will probably scream: "Nooooo!"
And going back to the topic: to me, Rogue One is kind of safe bet for Disney (which is logical, since they want their 4 billion investment to pay off ASAP). Granted, no Force, no Han Solo (except for a cameo, I heard today), no Luke etc. But we get all the familiar hardware (from X-wings to Death Star) plus a familiar plot, connected in a straight line with the holy ANH
. Who knows, maybe we're in for some surprises, like appearancess of Vader, Tarkin, Bail Organa or the Emperor? And in the same time, this "safe bet" is a strong signal to the viewers: look folks, we have so much more in store than just Skywalker saga. They need to do it early on, the sooner the better.
I think I'd do exactly the same that Disney just did.