Today, Turn 10 Studios announced that a new Forza game for Fall of 2012, called Forza Horizon. It is being developed by Playground Games, which is a new developer made up of devs from Bizarre Creations, Slightly Mad Studios, Black Rock, and Codemasters. Supposedly, it's a new take on the series, but nothing outside of the name has been revealed at this point.
There is a live action trailer, with an insanely obnoxious song, that was also leaked. It makes the game look very Fast and Furious.
There was also an announcement for a brand new expansion pack for Forza Motorsport 4 coming in May. It will bring 30 new cars into the game, specifically Porsche. This pack also includes 20 new World Tour events.
Like I said, Forza Horizon is slated to release this Fall, and the Porsche expansion for Forza Motorsport 4 is set to release in May.