A few things are getting clearer.

First it's already pretty clear just from the trailer that Abrams will be the "good student" (which might well annoy a few Lucas-haters) and he will perpetuate a great deal of the SW "mirroring"/parallels tricks and what Lucas saw as a symphonic structure.. Expect to have still a lot of elements forming variations on the same things we saw in the 6 movies, just like Lucas did that in the prequels. And oh... that BB-8 droid has more and more the hallmarks of being the new trilogy's Jar Jar/Threepio/Wicket as the butt of the joke/slightly annoying comic relief character.

The speculation that the new villain resurrects the Sith through some sort of quest for their legacy looks like it's probably dead on the right path.

The rumours that in the first movie a main character would look for Luke, or that Luke would seek that character because of her Force power and the awakening in the Force seems about right too. So it looks like we will have two "rival" quests, over the background of the Force "awakening" after being quiet for 30 years somehow.

I suspect massively that the teaser and trailers showed us mostly images from early in the movie, so my bet is this: For some reason - likely she needs him - Leia wants Luke, who she thinks retired to his homeworld of Tatooine. She's sent him old friends to bring him back. Pan down to Tatooine. The girl has not found Luke where he should have been, but before returning to the ship she investigate some rumor or oddity and surprise...remnants of the Empire have a base on Tatooine and they're not happy at all to be found. Somehow, a stormtrooper ends up following her. They have some adventures, it goes bad but the Millenium Falcon shows up for a rescue, and makes a daring escape... and we're introduced back to Han and discover the girl is his and Leia's daugther. Dad's not lost his touch with the Falcon..

Hyperspace... and back to report to Leia that Luke is not where he was expected to be. We're introduced to the third main, the X-Wing guy.

So basically we're echoing ANH (and a bit TPM) with a mission from Leia to find an old Jedi, which doesn't go as planned and brings a clueless youth out of there, and two old enemies clash for the first time in many years.

Some were still wondering if they'd drop the "family saga" angle for the sequels (a notion which didn't make much sense in the SW context, but there's always those people with weird notions about SW). The trailer makes it obvious that's not the case and that a female from the line of Vader is taking up the legacy. The odds that she's Luke's child are low, which is close to a confirmation that the trilogy's main character is Han's and Leia's daughter. Perhaps of interest is the fact Luke speaks of Anakin in the present tense, which suggests pretty much that he's still communicating with his Force-ghost, while the Sith Lord is rather after Vader's and Palpatine's legacy.

Another thing that's fairly apparent from the teaser and trailer is that they're reproducing for the sequels the male-male-female trio of heroes. No huge surprise there again, it's a central element of the series. They're reshuffling the deck again, though. This time the main is the girl, and it's pretty obvious she will be the trilogy's Solo (and thus more Anakin than Luke?) even if she's the Force user (and I tend to think she'll be the one). The role of the clueless youth caught in events too big for him it seems they've casted the antithesis of Luke dreaming to join the rebellion, ie: a stormtrooper who it seems will abandon the remnants of the Empire for the "Republicans". In the role of the older figure of the trio, we'll seemingly get an officer in an X-Wing squadron. A kind of older version of what Luke dreamed to become. The "big brother" figure won't be the rogue gunslinger figure this time, he'll be the responsible one. So he's the Leia, or a Han who actually follows the rules. If I go on a limb, I'd bet the daughter is daddy's girl, that daddy himself hasn't changed much at all but he's reliving his youth through his daughter, and that she infuriates Leia who I'm sure will be the political leader. So I think the girl will end up stuck with the X-Wing pilot one way or another, e.g. Leia will put him in charge of a mission over her daughter the daredevil, and her new "friend" the Imperial defector. That mission might well be to find/fetch Luke still. This time the girl will get the Falcon, but X-Wing man will be in charge.

We can guess the political context from the trailer and educated guesses from the 6 movies, and of course it won't be much like the EU.

I think we'll see that a vast number of worlds have thrown out the Empire (or been abandoned by it) and are now independent. Not really hostile to the ex-Rebels, but not interested either in joining them to recreate a pan-galactic Republic, which very few worlds cried over when Palpatine toppled it.

We'll also get a small "Republic" or Alliance led by Leia that still struggles a lot to convince more worlds to join it. I'm not even too sure they'll control Coruscant - I suspect they might have colonized Yavin or a planet much like it. I don't think Luke will have brought back the Jedi either, nor that the galaxy in general would welcome them back with open arms. Luke will have pulled a Yoda. A possibility is that the new Sith Lord will be a young man who's found Luke and tried and failed to convince him to bring back the Jedi and bring the Galaxy to Leia's republic, but Luke refused to even train him as a Jedi, and the guy decided to follow the trail of Vader and achieve his goals by bringing back the Sith instead.

Then we'll get the imperial remnants, likely not unified but perhaps on their way to be unified again. I think several regional Governors will have managed to stay in power and gather enough of the Empire's resources to maintain themselves in power, and resist attempts by rival ex-Governors to destroy/conquer them. My guess is that these mini-Empires are much too busy fighting one another, and on a local level maybe their independent neighbours too, for the war between ex-Rebels and themselves to have really continued, and I doubt Leia's alliance will have enough resources on their hands to have tried to finish them off in the decades post ROTJ.. so maybe a kind of cold war state that will soon heat up again. I wouldn't be surprised that one of those Governors is gaining momentum over the others, and that the new Sith Lord will want to be his new best friend, and turn the imperial remnants toward Leia's forces, and Luke.

I think the new trilogy will ultimately be the story of the Jedi's "redemption", ie that it will tell the story of how Luke's niece will bring back the Jedi Order, and how that new Order won't be much like the old one, at least not the old one as we knew it in its decline in the prequels, but rather something like the origins of the Order. Not much knowledge remains but what Luke knows from the Force ghosts about the mistakes of the Order, and I don't think Luke will live through the three movies, so my guess is that it will be up to the girl to define that Order and create it. It will seek to answer the question of why there should be Jedi at all, and why the Force needs them now that their post ROTJ purgatory is over. In parallel, the trilogy will tell the story of the attempt to resurrect the Sith. Not the Sith of the prequels with their rule of two, but to assemble as large a force as possible, allied with - or having taken control of - the imperial remnants. In the third movie the two Orders will clash, while in the background, with the return of Sith Lords and the remnants of the Empire coming together the systems finally flock to each side, and the two sides will go to war.

The new trilogy will complete the circle and end with a new kind of Republic on much more solid basis than that of the prequels, and a kind of redefined and reinvented Jedi Order. And of course no Sith.