Good start so far. I'd try to break up the green texture you've got for the grasslands. I can't imagine that the climate is the same over a 2000 km wide area so I'd expect there to be variations in the foliage and thus in colour. Also, for that scale the mountains are huge! But I realise they are representations rather than actual mountains. I'd also try to blend the bases of the mountains into the surroundings a bit more.