My idea about the film from the trailers and fan theories

Space: As it was hinted and mentioned by Vin Diesel, Michelle Rodriguez and Ludacris, the film will go into space, so my guess is that it will be part of the finale, when Roman and Tej meet up with Sean Boswell, Earl and Twinkie and use the pontiac ferraro with the rocket engine to find Cipher's HQ.

Jakob Toretto: I think that he is not a biological sibling to Dom and Mia, but he's their brother, but from a different mother. Cena has said that Jakob's not the main villain, but an "antagonist" similar to Hobbs in "Fast Five", but I see the movie ending with his true allegiance being left ambiguous, similar to Bucky in "The Winter Soldier", or could just be irredeemable.

Sean Boswell: since Lucas Black has returned to the series, He'll have more than one scene compared to what he had in "Furious 7". He, Earl and Twinkie were both seen at the table gathering in the superbowl spot with Dom, Han and the rest of the crew and his role could be much bigger in 10. Let's hope Twinkie has some banter with Roman and Tej

Han: Han's survival remains a mystery, but something tells me that he pulled a Nick Fury type of escape without anyone or maybe it's not Han his identical twin brother. I'd say his reappearance would prompt Deckard Shaw's return to the main series and possibly Owen's since Han could see Owen as responsible for Gisele's death. So 10 should have a Civil War plotline having Dom and Hobbs being pitted against each other once again.

Eteon: In the trailer we saw someone catch a car, we all thought that person was Dom but it's actually bodybuilder Martyn Ford, and if his character can catch car then it's could very well be Eteon working for Cipher, though there is no mention or proof that she's a supersoldier but she is good with technology and able manipulate it.

Nobodies: No sign of either Mr. Nobody or Little Nobody,but I doubt that those characters are done and couldn't rule out a cameo from one or the other and a good chance they'll show up in 10.

Brian's possible appearanceIn the Superbowl spot, we see a blue nissan skyline, which could point to Brian in the film and also at the table gathering in the Superbowl ad there is an empty seat so it could be for Brian (or maybe Jakob if he changes sides).