I think in the future there will be two Codemasters rally franchises: World Rally Championship (basically what Dirt Rally is now but with official WRC license) and Dirt (arcade rally, gymkhana, rallycross, Forza Horizon-type of open world stuff etc).
They can put classic rally cars in the WRC game. And if they deem that the 13 or so official rallies is not enough, they can always include classic rallies from WRC history such as Australia, China, Jordan or Poland.