Just as it's predecessors, it was always known that Forza Horizon 5 would be fleshed out with expansions. So far, nothing has been revealed regarding this, but it seems like we're not too far away from an official announcement.
As noticed by Wario64 on Twitter, the Steam page of the game was updated with a banner stating that the first expansion will have a Hot Wheels theme. Hot Wheels has been present with both an expansion and DLC since Forza Horizon 3, and have generally offered more playful ways of enjoying the game.
When this was noticed, the banner was quickly removed, but fortunately, Wario64 had already saved proof of this. We assume this expansion will be revealed during the Xbox & Bethesda Games Showcase on Sunday.