Here are my picks:
Wayne Pygram as Grand Moff Tarkin - Putting aside Pygram's little cameo appearance in Episode 3, Pygram has the cheekbones and the acting chops for the role. Seriously - go to Netflix, watch Farscape up to where the character of Scorpius starts making appearances, and then get back to me. (Though I'll admit that I'll have problems seeing him playing the role without imaging Tarkin saying "Kill her, Vader. Then we'll have pizza and margarita shooters.")
I agree entirely with Zac Efron as Luke Skywalker - he's a decent actor and has "boyish dreamboat" nailed down pat. Did I just say that out loud?
For Han Solo, I like Chris Pine, but if you're going to put The Stath in a Star Wars movie, this is the place. He can totally pull off a more smart-ass role like this one as well. Just look as Snatch and Lock, Stock.
Darth Vader - We need someone with a deep, imposing voice, so I'm going to nominate Vin Diesel to voice Darth Vader. Say what you will about the Riddick movies, but if I met Riddick in a dark alley, I would crap all my pants, and that's just from his voice.
Princess Leia - We need an actress who is young, attractive, and who is able to tell Vader and Tarkin exactly what they can do with themselves without seeming laughable. She's a bit older than what we're looking for, but Maggie Gyllenhaal is perfect for the role.
Obi-Wan Kenobi - Who to cast for the iconic old mentor figure. If I had a time machine, I'd take George Lucas' first choice - Toshiro Mifune. Unfortunately, we don't. So, instead I need the next best thing - Clint Eastwood.
Here's why: if you cast Mifune in the original Star Wars, the tone for the audience in the cantina scene (when Luke's being harassed) changes dramatically. At that time, Alec Guinness hasn't played a lot of roles which I would describe as being "action" roles. He'd still been in a lot of great movies, but nothing with a character who was projecting direct personal force. At the point of the cantina scene, Obi-Wan is just an old guy, and he's possibly the one in danger. With Mifune in the role, the scenario changes to one where we, the audience knows that the people messing with Obi-Wan (and Luke) have absolutely no idea who they are messing with, and if you take this as part 4 of a 6 part series, that's the expectation we should be getting. Thus, Clint Eastwood. He's an older actor now, at about the same place that Mifune was when Star Wars was made. He can also play roles with both the firm hand-but-soft touch that Guinness used, but also having the ability to become the biggest badass in the room like *snaps* that and then, with the same speed, turn it back off for when he checks on Luke. He's got the posture for both practically in his muscle memory.
C-3PO - This gets a little trickier. 3PO really needs to keep his kind of "butler out of his depth" role. I'd have the character be CG motion captured, and played by Rowan Atkinson: If he went full Blackadder, he could do it. If any of Mr. Bean ended up in the role, it would fail.
What do you think, sirs?