Although anyone who attended a recent pre-GDC event hosted by Microsoft in San Francisco yesterday should be under NDA, it seems there's plenty of loose lips out there as two new games bearing the successful Forza prefix have been all but revealed.
The below trailer (once again, assuming it hasn't been pulled by the time you're reading this) appears to be convincing teaser trailer for Forza Horizon -- a new game in the series that looks to not only diverge from the authentic track racing that the franchise is known for, but is purportedly under development at a separate studio as well. VG247 reports that PlayGround Games -- a UK studio formed in 2009 and home to a bunch of ex-Codemasters devs with games like TOCA and GRID on their resumes -- are handling development and if the trailer is accurate the game will feature street, rally and open-road racing (although the video appears to only be stock-footage, not the actual game) and land on Xbox 360 in Spring 2012 (US Fall).
As for the traditional Forza developer Turn 10, the same sources tell that they are hard at work on Forza Motorsport 5 and will continue to champion that line of the series for the foreseeable future.
Also purportedly out of the same event was the first look at Halo 4 and a new Xbox Live Arcade title called Fable Heroes, however details on both are scant at present so stand by while we wait for whenever the alleged embargo might end for those.