This is all speculation (only the timeline seems to have been confirmed by Lucas himself) but it's all entirely plausible and not that hard to have be done:
I doubt we'll see Obi-Wan. MacGregor would probably not do a TV show no matter what.
Yoda's a possiblity. After all, all you need is Frank Oz's voice. But I think that even if MacGregor would agree to do a small bit here and there, as well as Frank Oz lending his voice, I don't think you'll see them at all. I think Lucas and Co. will go with the initial impression that Obi-Wan and Yoda hide out after the events of ROTS and don't resurface into the story until when they appear in the OT.
It's very possible that Vader might be in it. All you need is James Earl Jones' voice and if he's willing to lend it to phone company commercials, I'm sure, for a price, he lend it to a TV show. Palpatine's very possible to. I don't think McDiarmid's in that much demand that he'd consider a TV show to be beneath him, or that's he's in the position to demand outrageous fees. Maybe even get the actor who plays Tarkin in Episode III. I believe he was made up to look like Cushing (via prosthetics) and if he doesn't sound like him, they can get a professional mimic to dub the voice in).
Boba Fett might be in it (anybody can be play him, just get Temura to dub the voice since no matter what the timeline, as long as it's after Episode II, he should sound like Morrison and not Logan) . As well as Jabba. Maybe a young Han and Chewie (Peter Mayhew would probably return so at least Chewie could have a role here and there). 3PO (at least have Daniels do the voice) and R2 (don't have to worry about Baker, even his work in the prequels is extremely limited and my impression is that he's there mostly for old times sake) are a definite possiblity
You might see a young Luke but I think it may be similar to that of Obi-Wan and Yoda. Their stories are finished until they resurface in the OT. But if so, it probably won't be hard to get the actor's playing Beru and Owen back.
Also, there could be Mothma and Madine (who I'm sure doesn't appear in the prequels).
The show could also deal a lot with the Tantive IV. Detailing its adventures as it faces, and narrowly escapes from, the Empire all accross the galaxy. There you could have R2 and 3PO (again, no problem in them returning, since all you really need is Daniel's voice) in supporting roles, appearing almost every week, and get the actor who played Captain Antilles in ROTS.
Also, I think Jimmy Smits might play a big role in the series. Maybe even the lead character (or at least a strong supporting character - maybe an Adama-like figure). He could appear in almost every episode. After all, he's had experience with TV before with NYPD Blue and L.A. Law. In fact, that's pretty much all he's known for since he's never had much of a film career. In fact, maybe that's why Lucas cast him as Organa. He was planning a TV show that would deal with the early days of the Rebels fighting the Empire even while making Episode II and hired Smits because of his past experience and success in TV (again, his only real success as an actor). Maybe the offer was put this way: Play a small part in II, a larger part in III, and that will segue into a weekly series after III's been finished and released. Smits, itching to get back to his former glory (having been a few years since he left NYPD Blue and realizing that his film career wasn't taking off the way he hoped, was eager to come on board).
So basically, the new series could be this: It's the rebels vs. the Empire taking place a few years after Episode III, centering mostly around Bail Organa and his Blockade Runner, the Tantive IV in which R2 and 3PO are serving aboard, as the ship flies through the galaxy getting into all sorts of Imperial entanglements. Often they're escorted by a squadron of X-Wings and Y-Wings and run into Imperials blockades - so major Tie Fighter vs. X wing space battles can ensue. Every now and then, we can cut back to Coruscant, the Imperial Capitol where the evil Palpatine plots the rebellion's destrution. Maybe Vader and Tarkin play a hand ons role or just stand aside while lesser characters (some imperial officer and his squad of stormtroopers goes out into the field).
So it all sounds entirely plausible. Basically they can do this. Have a TV show starring or having guest appearances by:
Vader (James Earl Jones' voice)
Boba Fett (Morrison's voice)
Jimmy Smits as Bail Organa
Stormtroopers (maybe a mixture of Morrison's voice and other actors as Stormtroopers since other clone sources can enter the mix).
Tarkin (the actor from Episode III and a professional impressionist to do Cushing's voice)
Ian McDiarmid as The Emperor.
Jabba (don't need to worry about any actor returning to do him, since with all the sound manipulation in his voice, they could probably get just about any actor to sound like Jabba)
3PO and R2 (at least Anthony Daniels returning to do the voice)
Mon Mothma (get the actress playing her in Episode III)
Chewie (with Peter Mayhew, but if not, they could have him, just get a real tall actor. Chewie's growl is mostly processed anyways so they don't need to worry about a specific voice actor).
Young Han (if they takes place after Episode III, Han will be from anywhere from his mid-teens on so they could get a decent young (late-teen-twentysomething) actor to play him.
Jar Jar (I sure hope not) and Watto (I wouldn't mind) could make appeareances as well (even Dexter Jettster)
So a Star Wars show could easily be done that could deal with major characters from both trilogies and not have to worry about awkward (a different actor playing a character we all know) cast changes (except of course when a new actor plays an OT character when they're much younger) since you could have so much of the actual actors return (mostley because they're not that famous or successful, and even if so, could at least lend there voice or do a cameo here and there) or have some characters from the OT be involved where it's plausible to get a new actor since there's no way they could return since they're much older and would have to play them much younger (this would only work for characters like Vader, the Emperor, R2. 3PO, Chewie, Boba Fett, etc). Also, the show could be somewhat of an anthology show. One week, they're focusing on the Tantive IV, dealing with Organa, Antilles and Threepio, and another week, it could all take place on Tatooine and involve Jabba and his smugglers. It could be similar in many ways to the Clone Wars cartoons - how that focused largely on Anakin, but veered off and told a story about Mace, or the other Jedi's facing Grevious.
So am I on the right track? Is this what they'll likely be doing? Carl?