I've always viewed Horizon as a spiritual successor to PGR.
Arcade flair is fine with me.
The problem is that 'Forza' is synonymous with the semi-sim genre now. MS/Turn 10 know this as the whole project was made in the first place to give the Xbox a proper Gran Turismo rival. Using the Forza name with an arcade racer would be an example of a company willing to diminish a brand's image for sales/$$$ rather than just being a legit attempt to branch out the Forza name.
Anyways, BADNED already said it isn't motorsports-related, so it wont be a 'PGR5'. I suppose you could argue it means it wont be a 'TDU' game either since that has racing in it as well. I have a feeling I'm going to be very disappointed once its announced. I dont get what sort of game would be car-centric, not include racing, and still be worth buying.