Probably not something that should be randomly added, but I think as an expansion like Hot Wheels was for previous FH games: the JGTC or Super GT.
-Racing with the classics: Supra R33, R32, NSX, etc etc
-Cars that can join (only those that were raced with IRL) are devided per season (year) of racing they were in
-The car can be tuned and customized as you the player wants, but need to follow some rules to make the playing field even as possible.
-The expansion features original Japanese racing tracks used in the JGTC
-Seasons can be focused on the JGTC expansion with new or classic cars that have been used during the JGTC or Super GT era
-The devs can even create or let the community create the original liveries of the racing teams that were popular during the JGTC era.
-Maybe the expansion can bring a team mechanic where people can join up and creat their own JGTC team and compete in weekly events/championships.
-Maybe people can drive up to the tracks 30-15 minutes before the race during weekend races, park their cars at designated carparks surrounding the tracks, giving them access to 1st person “camera stands” littered around the track, where they can “sit” in first person to watch the races while having a specialized UI that shows them the track, the cars position on the track and the leaderboard.
-Maybe Forza can introduce trucks to ferry race cars from the player housing to the tracks just for the fun of it.
Lots of cool things and extras can be done surrounding this gameplay mode while heavily honoring Japanese car culture and motorsport.
Lots of things can be added with something like this.