Its become a bit of a joke in the industry when it comes to Microsoft releasing a Forza game for the Xbox One and Windows format every year. Every alternate year you can expect the Forza Motorsport franchise, followed by Forza Horizon. In 2019 you should not expect a new Forza game at all. The developers are instead going to completely overhaul their current game.
Source: N4G PC Don't expect Forza Motorsport 8 next year, FM7 is being completely overhauled