Episode VII The Empire has been defeated & a New Republic has been formed. However the scars of the Empire & the decades of war remain. There has been a backlash against the notion of the Force from within the political sphere & the wider population. The rise of Palpatine, Vader & even going back to the Clone Wars & Dooku are all portrayed as a result of the infighting & corruptibility of Force users, namely the Jedi & the Sith. The claim is they are too powerful & even Jedi were shown to be seduced by this power. After Palpatine the New Republic leaders are paranoid about a powerful Force user ever holding a position of influence again. Despite the objections of a minority including Leia, it is decided not to reinstate the Jedi Order as protectors. Due to these fears it is hoped by many that the Force will fade out of all knowledge & existence. Despite these developments, Luke was determined to carry out Yoda's charge of passing on his knowledge of the Force. However he could not do so within the Republic. Though greatly saddened by this "new world" he decides to respect the law & leaves the boundaries of the Republic. He establishes a base at a far off remote location. He takes on an apprentice, maybe a few. Cutting to the chase, when a former Imperial officer shows up with the Imperial fleet (& a Sith or some nasty old Jedi) the New Republic is quickly overwhelmed. They soon realise that aided by the Dark Side this isn't an enemy they can hope to defeat via conventional means. Eventually the Republic appeals to Luke & co to save the day. An emissary is sent to him asking for help, maybe Han & Chewie. Of course some of the "next generation" will be also be key players in the fight. One of Luke's apprentices is seduced by the Dark Side. Also to up the personal motivation for Luke I'd have the bad guys either capture or kill Leia. That ought to fire him up. Another thought, perhaps the bad guys publicly discredit Luke by revealing he is the son of Vader.