I can only think of iracing ..
Assetto Corsa could be argued although it's not by design, more due to them taking so long to come out with AC2, and most of the post release support and updates have been by modders.
I don't see forza going this route and unless MS allows people to mod it. Personally I don't see this as a completely horrible thing because let's face it, track racing is pretty much the same exact thing every time and doesn't necessarily need a yearly update for a game that is based on it, especially when we're at them point of laser scanned tracks, not sure what else you can do with more iterations of a track racer when the hardware is essentially fixed.
The horizon series on the other hand can continue to add different locales and releases without seeming like they're just doing the same thing over and over.