If I had to guess? Call it Fast and Furious Stunt Park. Make it, essentially, a Horizon Rush-based campaign, with similar progression to Hot Wheels Park in FH5. You barrel through ranks, with Showcases (of sorts) and Horizon Rush events themed around and inspired by various famous stunts and sequences from throughout Fast and Furious's storied legacy to capstone each chapter of the expansion's story. The actual aesthetic of the island would be, essentially, one giant movie studio set in an Okinawa/Bonin Islands-inspired locale, seemingly tailor-made for Fast and Furious filming, with other references to F&F scattered about for the eagle-eyed fan. You'd get more Drag Meets and Drift Attack circuits--Jordan wouldn't have implied there was more than one Drift Attack circuit outside of Shimanoyama if PG wasn't thinking about more permanent additions, while the routes would perhaps start feeling more cinematic than 'realistic', with more opportunities for Skills in the races themselves. Basically taking what worked from Horizon Rush and translating it into actual races.
The Horizon Story would, like Hot Wheels, essentially be a guided tour of the franchise's filming history, while the story justification would perhaps be Horizon getting contacted to help put together a documentary on the franchise in a way only Horizon can--by giving its Legends free license to tear it up the way Dom and family do. Iconic F&F cars (that aren't the Tokyo Drift Mustang we already have) will be rewards for progressing through the DLC campaign, while the Horizon Story perhaps gives out a bunch of up-and-coming speed demons and/or future stunt cars, like the C8 ZR1. Heck, maybe you could even end it with revealing the 'hero car' for Fast Forever and giving it to everyone who owns the expansion for free, just to drive up hype for that movie.
EDIT: Oh, crud, brainwave hit me right after I sent it. Maybe crib from Rally Adventure and divide Mei, Kyoko and Jordan into three separate mini-campaigns just like that expansion did, each one focusing on a specific aspect of Fast's movies--one focusing on the initial street racing movies, one focusing on the skill-based driving in the middle of the series, and one focusing on the stunt-driven supercar-glazing modern day Fast and Furious movies with their more ridiculous premises and stunts. Essentially you'd end up bringing all three together as the 'Horizon Family', as corny as that sounds, to symbolize how Fast isn't just one thing or one genre of movies.