Okay, so here are my thoughts on the likely back-story of the movie:
BEWARE - Possible Spoilers!
Watch the trailer before reading this!
My guess is that the galaxy sort of fell into anarchy after the end of the Civil War. There are several different factions but no-one has been able to get enough of an advantage to reunite the galaxy, Empire or Republic. I suspect that Kylo Ren is part of a faction that has emerged that is determined to reunite the galaxy, no matter the cost. He has latched onto Anakin's dream (as he expressed to Padmé in 'Attack of the Clones') of a galaxy of order and justice no matter what it takes and how tyrannical the means and wants to see it through - What TVTropes.org would call a 'Templar' or a 'Well-Intentioned Extremist'. As we can see, he's chosen the Dark Path to do so.
My guess is that, in the post-Imperial anarchy, our heroes have mostly ended up as exiles. In the 30 or so years since Darth Sideous and Darth Vader's deaths, the whole story of the Civil War has become entangled with the various factions' propaganda and political agendas. It has come to the point that not everyone really believes that there ever had been a 'good' uprising against the central government, nor does everyone really believe that there were once Space Wizards capable of calling on something called 'The Force'.
Over-ideological embarrassments to some sides because they won't play the politicians' games and criminals to be killed to others, our heroes have been totally marginalised. It looks like Han and Leia have been trying to keep at least an echo of the original Rebellion and its goal of restoring the Republic alive. Meanwhile, Luke has never been able to train any apprentices (or if he has, only a vanishingly small number). This is either because it has never been safe for him to act openly or, perhaps, because General Order 66 was very successful in wiping out nearly all the Force-sensitive bloodlines except for a few favoured by the Emperor himself. In either case, it seems that he now lives at least in part a hermit, possibly in that temple we see on the planet where Han and Leia are maintaining their own faction. Now, Finn and maybe Rey come forwards and, perhaps, Luke and Leia have finally a chance to pass on their legacy.
tl;dr - The Sequel Trilogy will be about the two conflicting legacies of Anakin Skywalker - A dark, destructive, nihilistic order verses one of light, freedom and healing. The irony is that Rey, Finn and Kylo all have the same objective: to end the destructive conflict and restore peace to the galaxy; they just have very different ideals about how this needs to be done.
Possible Movie Plot
Okay, here's my call. It's random and it's probably wrong:
- Film starts with Snoke speechifying to his troops. Phasma and Kylo then lead the troops on a raid against some helpless planet 'to impose order', which turns into a massacre. The sequence ends with Finn having a very bad reaction to events when back on his command ship;
- We cut to Rey on Jakku, doing her scavenger thing and basically showing that she doesn't want to live in this desert community descended from the survivors of a long-ago space battle;
- Finn, sickened, steals a TIE Fighter and shoots up the landing bay of a Star Destroyer and escapes only to get shot down over Jakku. After wandering for a while, he arrives in Rey's hometown. She introduces him to 'The General' (Han), who she describes as a 'crazy old man with a great line in stories'. However, Han gets warning that The First Order are on their way to attack the planet; he tries to persuade Rey and Finn to leave with him;
- The First Order attack Jakku, probably for no other reason than to 'impose order'. Han and Chewie have the Falcon nearby and somehow they manage to turn the raid back single-handedly, shooting down a Star Destroyer in the process; Rather than take the two youngsters back to the settlement, Han announces that he needs to take them to 'somewhere safe' and the Falcon jumps to lightspeed;
- Rey is doing her stint in the Falcon's cockpit when something (The Force) impels her to drop back to sublight in the Endor system. Han grumbles but, thanks to his long association with Luke, has learnt to trust hunches like this. Finn and Rey encounter Kylo and some of his fellow Knights of Ren. There is a brief battle in which our plucky heroes are completely outmatched; Finn is knocked out by a falling tree and Kylo orders the other Knights to kill them whilst he goes to take what he came for - Vader's helmet. There, he encounters Luke and R2. It is quickly clear that, although far older, the Jedi Grand Master is skilled and powerful enough to sweep Kylo aside like a gnat, so he pulls a typical Sith trick and tells him that he needs to decide what is more important - the Sith relic he is guarding or the two Resistance fighters that he has left with his colleagues to execute in whatever way amuses them. Luke makes his choice (the only choice) and swoops in to take out the Knights whilst Kylo takes Vader's helmet and flees, leaving his colleagues to their fate at Luke's hands;
- Han flies the two battered two youngsters to the Resistance base on Yavin-IV, where he meets up with Leia for the first time in decades. Han then takes the two youngsters to Luke's hermitage. It is revealed through exposition that Rey is Luke's daughter and that Han took her into hiding on Jakku when the attempts to unify the galaxy after the Civil War imploded and the leaders of the Rebellion became wanted outlaws by almost all the factions. Luke implies strongly that Kylo Ren is someone known to him personally but he is reluctant to be precise on how;
- At this point, the Starkiller arrives in the Yavin system. Initially, the First Order attempts to take the planet by a surface assault. However, the Resistance puts up a tremendous fight both in air and on the land (including Luke and Leia holding off a whole legion of Stormtroopers and vehicles on their own at one point). However, Leia knows that they are fighting a losing battle: no-one knows where the Starkiller's 'small thermal exhaust port' is, unfortunately. The Resistance's ships start making their escape but Poe volunteers to lead a rearguard to hold the line until the last possible moment. The sequence climaxes with the Falcon and Rey's X-Wing jumping to lightspeed just as the Starkiller blows up the Yavin system;
- First epilogue is Kylo monologuing about his goals to Poe before using the Force to strip out all the secrets about the Resistance that Poe has in his mind. As a few other Knights drag Poe's (presumed) corpse out of the room, he delivers the "I will finish what you started" line to Vader's helmet;
- Second epilogue is somewhere else where our heroes are taking stock. Luke delivers his 'The Force is strong in my family' line to the youngsters. Finn, who wants a cause, is very ready to accept Luke's offer of training. Rey is initially unwilling to believe that the 'magic' she's seen Kylo use to such evil effect could have any good in it. Leia reminds her of all the intuitive guesses and luck she's enjoyed all her life; the thing that kept her alive when she flew an X-Wing during the second Battle of Yavin on more than one occasion. The Force is calling to her; she needs to let it in. Rey finally agrees. Leia gives Anakin's lightsaber to Rey but the girl ceremonially gives it back, noting that Leia had made the best use of it to date and that she would make her own, when the time comes;
- The film ends with them watching the stars from the roof of what is now the closest thing the galaxy has to a Jedi Praexium.