Sometime after the events of Episode VI, Luke started to have visions about his past and present. He was unsure of what it all meant. He started to look into his past and discovered some dark things - how his father was created by evil.
With his increasing power and uncertain of his own intentions Luke becomes a recluse on the Sith world of Moraband - where his father's conception took place. His visions worsen and he begins to see the return of 'Darth Vader' both masked and as a deformed man. Burning villages, laying waste to cities etc. He wrongly believes this could be his own future.
I believe this 'Darth Vader' will turn out to be Adam Driver's future instead. At the end of the movie Driver will suffer a horrendous injury and be ''saved' by Plagueis and become his apprentice. He will wear the Darth Vader suit from then on (there is no way Disney wouldn't have Darth Vader in a Star Wars film).
Before this happens, Han Solo will be killed (either by Luke or his own Son) but not before asking Luke to redeem his son like Luke did his father. Luke will then train Kira and with the help of Boyega's character and maybe Serkis/Gleeson, the 3 new central protagonists will seek to bring peace to the Galaxy.
It maybe possible that the backdrop to the story will be that the galaxy has been in a state of 'Cold War' for many years. The Empire will have been awaiting its chance to strike once again and VII opens at the beginning of that attack. I think the tone of the movie could very much be a mysterious cold war type thriller mixed with the classic Star Wars western space adventure.
Boyega could be a spy who discovers something grave about the Empire (and perhaps Plagueis' plan) and seeks to find Luke Skywalker. He first comes across Kira (ahem) and then they set off together to find Luke - the only person who can stop the Sith. The other villains could be the old Sith Inquisitors or just dark force user types who serve the Empire.
Just my two cents there...