Ultima Sports, manufacturer of cars like the Ultima GTR, posted on their Facebook page (via Worldsfactory.net) a partnership with Microsoft to possibly bring another Forza title later this year.
The post explains the partnership with Microsoft and their vehicles in Forza Motorsport 4 and Forza Motorsport 5. Ultima Sports then goes onto explain a newly agreed contract to include Ultima Sports in a new game to launch later this year. The manufacturer deleted the post, but it can be read through Google Cache or in the statement below.
Our relationship with the Microsoft Corporation continues after the success of being included in Forza Motorsport 4 we are now also included in the latest Forza Motorsport 5 game which was launched in late 2013.
We have also recently agreed a new contract with the Microsoft Corporation to be included in a new video game to be launched later in 2014.The Ultima is proving to be very popular and is providing excellent worldwide publicity and recognition for our marque.
http://www.forzazone.com/forza-motorsport-forza-horizon-car-list-manufacturers/1102-ultima
In July of 2013, Playground Games, developers of Forza Horizon, posted multiple job listings on their career page looking for new talent. The positions referenced experienced developers with knowledge of car handling and interest in "cars and car culture."
A LinkedIn profile from a Playground Games vehicle artist also mentioned an "undisclosed next-gen Forza project."
Turn 10 recently released Forza Motorsport 5 for the Xbox One, and Playground Games has yet to release a game since Forza Horizon in 2012. Both studios worked closely on the Forza spin-off, but have not discussed future projects since.