I don't think that Forza Horizon should add motorcycles as the physics of cloth, momentum, and collision are not really that well implemented even in dedicated titles. Plane races are devoid of impact without weapons and their addition would probably be pointless. Water racing is cool, but it is more suitable for arcade racing games, which Forza Horizon is really not.
What I think Forza Horizon 6 should do is greatly improve Road Series events, add some rival personalities to their Street Races, and add a somewhat elaborate racing track with go karts. I also think that they should take a step back with regards to destructible trees, rock fences, and rail guards and instead work on car deformation (maybe add a car derby event). I think it would also be neat to add a second interchangeable avatar to ride along and cheer the player on or just look cool. I would also be interested in them changing the drone mode to be a free-roam helicopter and any discoveries prompting landing and collection. Also, as weird as it may sound, I think they could play up the tourist angle and add famous restaurants, hotels, and casinos for their next destination.
P.S. I hope they remove the hearing aids, prosthetics, and gender nonsense from their next entry. They add nothing but a reminder of the absurdity of life. Also, more authentic accents for locals. The proper pronounciation of Mexico was butchered in FH5.