Forza Motorsport is revealed at the Xbox Games Showcase
During the Xbox Games Showcase, Turn 10 did reveal that they have been working on the next entry in the Forza Motorsport series, via a teaser trailer, which is below. The downside, that is all they showed.
Phil Spencer announced the reveal but prefaced it by saying that “They are early in development" on the title, the problem with that is that it has been 3 years since the last one released, so you have to ask just what have they been doing.
Sadly, there is no word on when the game will show up again, so just replay the trailer above and hope it is not to much longer.