Rogue One has a lot of ground to cover as a standalone story that also acts as a direct prequel to the original Star Wars. We already knew Darth Vader was going to be around to provide continuity. Now we seem to have confirmation that there's going to be another Imperial making an appearance, and he's even more welcome than the Dark Lord of the Sith.
As everyone and their mother has pointed out, one of the problems with Rogue One is that we basically know how everything has to work out at the end of the movie. The team will obviously succeed in getting the Death Star plans, although we don't know who might die to accomplish this. But given that Director Krennic (the fabulously caped Ben Mendelsohn) isn't in charge of the Death Star when A New Hope begins, it's probably safe to guess that Krennic doesn't survive his debut film.
Of course, it's Grand Moff Tarkin who commands the Death Star in A New Hope, and he's the one who seems to have been revealed earlier today making an appearance in Rogue One (thanks to a weirdly spoiler-y but 100 per cent official TV spot). While Tarkin could simply show up at the end to take control, I pray that he's much more involved in the film -- and here's where things get a little more spoilery if you haven't read Catalyst: A Rogue One Novel.
It basically boils down to this: Wilhuff Tarkin and Orson Krennic loathe each other. Of course, in theory, they are both Imperial officers working for the glory of the Emperor, so they have to be subtle about how much they want to murder each other. They're like passive-aggressive super-rich WASPS who have to plan a charity gala together. It's almost beautifully bitchy. I would not be surprised if Tarkin secretly thought the Death Star was a stupid idea and he only takes control of it because he knows it would piss Krennic off... even if Krennic's dead when he does it. It's the kind of petty thing he'd do.
Here's an exchange the two have in Catalyst that gives you some idea of how these two interact with each other:
"Must be nice being a legend in your own time," Krennic said.
Tarkin vouchsafed a tight smile. "It's not a position one simply applies for, Lieutenant Commander."
Krennic returned the look. "I'll bear that in mind. Thank you for making time to meet with me."
"Laid up for repairs as we are, I appreciate every opportunity to relieve the monotony."
Krennic glanced around the enormous hanger. "Odd, it's almost intimate compared with what I've grown used to. Still impressive, though."
The sarcasm wasn't lost on Tarkin. "Of course. Your big ball in the sky."
I would watch a whole movie of just these two. It is legitimately upsetting that Peter Cushing and Ben Mendelsohn won't ever actually share the screen together.
Their interactions give a lot more insight into the Empire, which has always been mostly "generically evil" with the Emperor being a manipulative power-hungry tyrant. The old Expanded Universe made the Empire speciesist and sexist, making it clear that only human males were really welcome there. Hell, in the old canon, Admiral Ackbar was enslaved and given as a gift to Tarkin. And then the old fish kicked the Empire's arse after he escaped.
Anyway, Tarkin and Krennic are both older men with distinctively sharp faces and British accents. They're both manipulative and both rising stars in the Empire. That is where every possible comparison stops. Tarkin comes from a wealthy family and his power comes from a distinguished military career. Krennic is from a working class family and he's an engineer who can't rise through the military ranks no matter how much he wants it. Tarkin gets along well with Vader, Krennic doesn't. Krennic takes everything personally. Tarkin doesn't have anything approaching the ability to care about what others think.
Hell, the difference is even in their outfits. Tarkin has the simple grey of any Imperial officer. Krennic has all that white and the big old cape. He wants to look as impressive as he feels he should be, while Tarkin just has to talk to send chills down your spine.
That contrast -- the man who had every advantage and the man desperate to prove himself -- makes Krennic a much more interesting villain. So while Vader being around is cool, Tarkin actually does a lot more for the story, since he has a connection and competition with Krennic. Having Vader appear is nice for continuity between Rogue One and the other films, but he doesn't bring nearly as much to the film as Tarkin does.
Since the presence of Tarkin rests on effects magic, I doubt he's going to be in Rogue One nearly as much as I want him to be. But think about all these things every time Krennic shows up or in the few shots of Tarkin, because it deeply enriches the story Rogue One is telling.