Remember that the landmass the expansion takes place in is really not a problem; Land can be risen from the sea, discovered from mists, introduced into our dimension through bronze shenanigans, created with magic/fel/void, a gigantic old god can rain from the sky and its body become a continent, etc. There are really zero real limitations behind the argument "but there is nowhere left for the centaurs to set up a base!"
Personally I still hope we will one day get the mongrel horde in some fashion, I think that can make for a far more interesting narrative than the void.