I think people are seriously over reacting and over estimating GW's willingness to completely destroy thier fluff for no gain.
Some characters will die, maybe others come back, and others might be invented, but I don't think the final damage will be enough to end the story.
Also, I doubt they will even mention the other human countries of the world like Cathay and such. They spent so meny years ignoring them completely I think they belive most players won't even remember they exist.
And if they would touch upon that it'll have to be something other then invasion. As Chathay is massive and has huge armies at its desposal, even bigger then the Empire from what I remember.