Update:
Microsoft has confirmed all of the below, revealing that Forza Motorsport 6: Apex will have 60 cars and 12 themed events. Although reportedly it is not ‘microtransaction-driven’, despite being free-to-play. The Porsche expansion has also been confirmed as coming out today, with more information on the Xbox Wire blog.
There doesn’t seem to have been a lot more details than that, although Forza developer Turn 10 has confirmed that in the future all Forza games will be made concurrently for PC and Xbox.