According to Phil Spencer, "I'm excited to share that players will get to celebrate 25 years of Xbox with a new Fable, the Next Forza, Gears of War E-Day, and the return of a classic that's been with us since the beginning." The speculation is that, since the most recent Forza was Motorsport, we could be seeing the next Horizon. Additionally, they're stills adding to and tweaking Forza Motorsport (2023) so I'd imagine it'll be a while before we see a new Motorsport.
Of course, there's the possibility it could be a new Forza series altogether. There's no evidence supporting the addition of another series, except that once upon a time there was a vague reference to the possibility, but the wording of "new Forza" instead of specifying Horizon could be intentional so as to acknowledge the upcoming game without giving away the detail about the new series.
