Turn 10 is currently not actively working on Forza Motorsport 8. During a livestream on Mixer the dev said they are working hard on improving Forza Motorsport 7. Creative Director Chris Esaki said, that "the entire team, 100%...the entire Forza Motorsport team is working on Forza Motorsport 7. What that means is, we're not going off making Forza Motorsport 8. We're actually not making a grand new expansion. We're investing in the motorsport experience you have right now." The livestream revolves around smaller updates and paid DLC content coming the next few months.
We still assume we're getting a new Forza Motorsport game in October next year as surely much of the work carries over into the next game anyway...