He brushed it off saying casting will begin once they have scripts...

Of course what he doesn't say is that if they intend to bring them back, they have to sign something with these three, a retainer of some sort for now, before they actually write the characters in these scripts, and unless it's for inconsequential cameos that can easily be cut, it's probably already negotiated or being negotiated now (at least for the one(s) who have more than a cameo role.. if they're toying with the idea of having Hamill appear to the extent of Guinness in ANH, for instance).

Then Fisher's agent has released the explanation that she was joking about returning (she has an habit of pulling the leg of SW fans, though I don't think she really was this time, nor was the far more reliable Hamill).

But it's a badly hidden secret (and not really meant to be one) anyway: part of Lucas "sale's pitch" for LFL was to approach the original cast and write down story treatments incl. those characters based off vague notes/ideas from the 80s revisited and no doubt very much changed post-prequels as his original sketches predated making Vader Anakin Skywalker and Leia Luke's twin.

Kathleen Kennedy as CEO, treatments being developed as scripts and more or less secured deals of some sort with some of the original cast is the silver platter style bundle he intended to attract a potential buyer. To now kick start the nostalgia with "leaks" and denials is probably just some icing on the cake routine. Whether LFL got Fisher and Hamill on a kind of "retainer" contract before being sold is anyone's guess. I tend to think they were signed by LFL and are now negotiating with Disney (that could just pay them and not sign them to be in the actual movies, but if they had doubts the positive reception this got in the fanbase probably ruled that out).

He's pretty much bound Disney to those plans when selling - if they chose to go forward with numbered episodes anyway, as he sees this as his legacy and the only way he could solidly influence where he was going with remaining "numbered episodes" (Lucas never closed the door to unnumbered spin-offs and such, early on he opened the door wide for those). It's also pretty clear Lucas believes the return of older Luke, Leia and Han is what the SW fanbase wants (but he's no doubt very aware they don't have the same aura among the vast and much younger generation of prequels/CW series's fans, so expect prequels tie-ins too. No, not Jar-Jar. Disney would have to be insane to bring the Gungans back in anything but background cameos).

It's not likely to be central roles for the big trio, though. Hamill once revealed Lucas intended to write the characters off if one of the original trio refused to return.

The well-timed Lucas "leaks" (to get people to talk SW in the pre-prod. phase when Disney has nothing to tell beside hiring a director and such...), the actors's, the Disney half-denials (it's basically "it's not true yet, but it might be later"... it all smacks of engineered "leaks" to create and then milk a "buzz", something both LFL and Disney are very, very good at. They'll waltz around the issue for a few months before making the full announcement (but if the original cast wasn't returning, Lucas would never have "leaked" this). They're also very likely probing this way the fanbase's and general audience's feelings about the return of the trio before deciding how much to involve these characters in the trilogy (and the "vibe" about the rumors is fairly positive in SW communities). They're also probably testing the water for the marketing power of Fisher and Hamill, as a prelude to using them as "ambassadors" for the new movies. Do their re-appearance talking SW get the media and fans excited? Answer: oh yes. The interventions of Fisher/Hamill escaped the SW fans and easily made it as "news" in the mainstream media. So expect them to be all over the place once they're ready to "announce" the new movies will feature a return of the original cast. They will likely ground all their "nostalgia" campaign around those two (which much, much rarer comments by Ford).

Pure speculation on my part, but my guess would be that Hamill's role is likely to get enlarged in the actual writing of the script(s). I imagine Leia will be more background, in the vein of Palpatine in TPM/AOTC, while Ford might be "tangential". Ford could get anything ranging from occasional cameos not to leave him out to a proper "last Han Solo adventure" in the vein of Connery's role in the third Indy (the "inappropriate" old man is a role Lucas is very fond of, and a Kenobi-like Luke vs. a old Han that would still be his old self if he could could offer good, funny contrast). In this vein, Lucas also said his treatments are loose, with plenty of room to expand stuff or eliminate elements.

My guess about the Ford situation is that Harrison Ford gave Lucas a tentative "go" to include the character in his treatment, but his actual return depended on the schedule, director and such.

The trio could also well play their larger (or only) role in the first movie. It would be typical GL to use them to bridge things before movies 2-3 bring the new players fully to the center stage.

As for the story, it's quite uncharted territory. The SW fan sites seem to take for granted (maybe Lucas has said something about it I missed) it will bypass the Expanded Universe. As much as it will infuriate the EU fans, it only makes sense. It would be a terrible idea to burden the new trilogy with decades worth of complex back story and characters (especially since the EU and its characters have gone through many eras/crisis). Either it would all be made irrelevant and full of inconsistencies (the whole "it did happen, but it's referenced only through one liners nods), which would open the door wide to EU fans bashing, or it would force the movie creators to introduce tons of back story and info dumps for everything from the EU back story they want to use. If you want to use, say, Mara Jade's past in any way, you need to bring it up so it makes sense to the movie audience who never heard of the character and the tons of novels in which she appears, and never will. And it will be the same for all EU characters, technologies, plot points they wish to use. The Solos/Skywalkers new generation in particular has way, way too much back story to be used.

Much easier and sensible to wipe the slate clean and "reboot" the post ROTJ years, maybe give the EU fans a few Easter eggs or a character or two.

Lucas himself always said the EU didn't take the direction he (loosely) envisioned. At first it was because he didn't want to fully close the door on possible movie sequels (though by 1983 he no longer had plans to ever make any) and he restrained the EU to covering the early years post ROTJ and the final fall of the Empire (himself intended to jump ahead 20 years to an early "New Republic". As the year went by, the EU was successful and Lucas was sure he'd never make sequels, he let it... expand, and occasionally guided them within the logic the EU had taken. When he wrote the prequels, he didn't hesitate to simply wipe out everything said about that era in the EU. He almost certainly did the same with his new story treatments.

As for the themes he considered playing with in eventual sequels (which Lucas described back in 1983 as "the necessity for moral choices and the wisdom needed to distinguish right from wrong", he ended up incorporating them a great deal in the prequels and the story of Anakin (mostly through the characters of QGJ, Padmé, Obi-Wan and Yoda), in the background of the story/themes he was speaking of back then for the prequels (so essentially, Lucas has rewritten the prequels also digging in his sequels ideas liberally. It's around that time he started saying the whole thing about having planned 9 movies was just a "media thing".).

The real story is more that back in 78-79, young Lucas was terribly excited by his franchise and sketched plans for a fall of the Republic/CW era (an Obi-Wan/Palpatine trilogy) and a "redemption" trilogy (and was a little too talkative with the media, presenting vague plans as very concrete plans), already seeing himself as the exec producer and "puppet master", envisioning the saga as a playground for other directors. Then ESB happened and with it its reality check: it turned into a nightmare. Filming went wrong, he fought with the director constantly, he lost the writer to cancer and had to finish himself, he got sick, the producer let the budget double. It's around this time Lucas (and the original ESB writer) decided Luke wouldn't be the failed hero who fell to the dark side in a misguided quest for revenge for his father's murder. They decided Anakin would be this failed hero. That let them the possibility to end the story with one more movie, without introducing "the other", Luke's twin that would triumph over the Empire and rebuild the Republic... but without fully closing the door. That's finally how it ended up (in part it was the result of a costly divorce that it took many years to Lucas to recover from financially, it's the EU, the VHS/new theatrical run, the success of ILM that put him back on his feet, lead to the restored trilogy which in turn decided the fate and financed the prequels).

After ESB Lucas decided to end it with ROTJ, and it crystallized in turn the story frame of the prequels. It was no longer the story of Anakin's and Obi-Wan's failure ending with a Clone Wars movie (with Luke appearing as a young child that an Obi-Wan on the run ended up hiding, while presumably Yoda hid "the other", it was re centered on Anakin Skywalker's downfall, and ROTJ on his redemption. Leia got rewritten as "the other", and it opened the door to Padmé's character (she first appeared in an early draft of ROTJ after story meetings in which there was pressure Lucas's way to mention the mother character: Ben/Yoda told Luke his mother had been a close ally of Kenobi who failed to see Anakin had turned to evil in time, and he killed her. Lucas however changed his mind about the wisdom to reveal the fate of the mother he now wanted to be the Leia of the prequels, and he had all of that cut in later drafts). Exit Luke's fall and his redemption by his twin, and making him the sequel trilogy's main villain. Exit the sequels, Lucas has managed to pack the whole thing, or most of it, into six movies.

God knows what ideas Lucas came up with, but it's fairly widely assumed those treatments were written fairly recently, likely around the time he decided to pass the torch to Kathleen Kennedy and to develop the idea of a new trilogy as "live bait" for a buyer.