Story theory (based on the trailers)

So we know that it begins with Dom and Letty living private lives off the grid with little Brian. But somehow the reoccurrence of Cipher and Dom's long-lost brother Jakob forces them back into action. The crew - Roman, Tej and Ramsey reunite with them for this new mission assigned by Mr. Nobody. Jakob's plan is to steal a weapon so dangerous that it shouldn't exist for another half century, but Jakob's plans are in motions and is in cahoots with Cipher.

The team that arrive some Island where hell breaks lose and they fight Jakob and assumingly his team. Jakob steals the weapon and Dom and Letty chase after him. Jakob then escapes with help from Cipher. After the mission Mia will come in, with Brian staying at home with his kids. The team would then split up with Dom, Roman, Tej and Ramsey heading to Edinburgh (or London) and meet with Magdalene Shaw to help them track Jakob. Letty and Mia would then go to Tokyo for some reason and meet with Elle (Anna Sawai's character) and they fight some of Jakob's men, but are then saved by Han (who is alive), I guess Elle has some connect with Han. In Edinburgh (or London) Dom heads into town and fights Jakob through the streets. In the aftermath, the crew reunite in the bunker with Dom and Han reuniting and Elle joins the crew in their mission. Eventually Jakob and his men capture the team and they fight Dom taking him down. Eventually Jakob will somehow turn against his men as seen in the trailer and join Dom in his mission. Roman and Tej would then meet with Sean, Twinkie and Earl for the "space" mission with the rocket car, to possibly stop the missile. In Tbilisi, the rest of the team Dom, Letty, Mia, Han, Ramsey and Elle along with Jakob go after the mercenary team and convoy truck using the magnet devices. Jakob then fights his men (including bodybuilder Martyn Ford) on top of the truck and Dom, Letty and Han manage to tilt the truck which turns it upside down and slides down the streets, Dom goes inside it to probably for Jakob and tries to stop it, but sees Cipher in her plane who is about to kill him but he jumps out and the truck hits her plane (probably killing her or leaving her badly disfigured). I guess Jakob will then part ways with Dom because Cena has said that Jakob is not the main villain (possibly just an antagonist like Hobbs was in Fast Five) and will leave it to the audience to decide. I'm guessing that Jakob will betray his team but probably not team-up with Dom because in the trailer it shows Jakob inside the drivers seat of the truck and both Han and Dom's cars turn it over and Dom then goes inside. And I guess Jakob jumped out.

Lastly their is the BBQ at 1327 where the crew - Dom, Letty, Mia, Roman, Tej, Han, Ramsey and Elle all reunite with Sean, Twinkie, Earl and Rico Santos (Don Omar) joining them. In the trailers there is an empty seat at the gathering (so could be for Brian? Mr. Nobody? Tego Leo? Jakob? I guess time would tell).

EDIT: From recent trailers and BTS content, it seems that Jakob is driving the armored van that Han, Mia and Elle are in, becuase in one featurette it's show Mia grabbing someone, and I believe it is Jakob. Also I think the line from "Dom's story" trailer "You are my brother, but you will never be family" is probably before the BBQ scene.

-Flashbacks
Flashbacks are confirmed with Finn Cole playing a younger Jakob, and actors for young Dom, Kenny Linder and Dom's father are confirmed with younger actors for past franchise characters. These flashbacks will be of when Dom's father was killed and how Jakob became what he is.
Vin Diesel has said that there would a flashback to the first race of the first movie, but from a different perspective (Jakob's, considering we didn't see him, maybe he was keeping tabs on his brother in secret).