While we have been watching for more clues, the latest confirmation that Forza Horizon 2 was in development came from a certain Supercar Manufacturer who admitted that they had signed a new contract with Microsoft for licensing their marque for use in the game. Beyond that, additional clues from developers themselves point to Forza Horizon 2 being in development.
Our sources tell us that Forza Horizon will be announced early this year, but it has not yet been decided when. We suspect that it will be before E3, but by E3 at the latest. It will ship yet this year in the late Fall. As for the platform, while it will be (of course) an Xbox exclusive, we suspect that it will only be released on Xbox One and there will be no Xbox 360 release; but you never know if that might change.