The location is obviously, an island, a fictional one specifically, and there are indeed treasures to find including both cars and actual treasure chests that will thoroughly bolster your in-game bank account. The island will also feature foreboding weather conditions such as furious thunderstorms. There will also be more peaceful times as the Aurora Borealis will sometimes be visible. And there will be more events to participate in, including "Trailblazer" features that leave the route to the finish line entirely up to you. The developers also brag that Fortune Island features "the largest paved mountain switchback road in Forza Horizon history."
Fortune Island adds a whole new batch of cars previously unavailable, too. They include off-roaders such as the Ram 1500 TRX Concept, Exomotive Exocet Off-Road, Funco Motorsports F9, and Ken Block's Gymkhana 8 Ford Fiesta ST RX43. Road-oriented cars include the 2019 Lamborghini Urus, 2018 BMW M5, 2002 Koenigsegg CC8S, 2012 Lamborghini Aventador J, 2010 Saleen S5S Raptor and 1953 Morris Minor Series II Traveler.
The expansion is available as part of the Forza Horizon 4 Ultimate Edition, or bundled with the Expansions Bundle for $34.99 or the Ultimate Add-Ons Bundle for $49.99. It will be released on December 13, 2018.