One a year? Seriously, Turn 10 needs to put more time into making a game that steps things up significantly. It's not like there is another racing game (on the MSFT platform) that even comes close.
It's been 2 years since Forza 5, or will be assuming they ship this fall/holiday. Horizon 2 was made by Playground Games in the UK, not Turn 10 (though I'm sure there was some assistance, etc).
I'm fine with having the two franchises alternate every couple years, they are different enough that having one racing game a year isn't that big of a deal.
That's not to say I'm as excited about a Forza 6 as I might be a new Rallisport Challenge, but that's a whole different matter.