However, since White/Black Kyurem are part of the main plot it's unlikely that will happen.
You could also use that very reason to argue that the respective dragon will
appear in BW2. That's what I personally believe. I mean, why not, right? If Kyurem absorbs one of the other dragons, wouldn't it make sense that said absorbed dragon will be part of the plot in some form or fashion?
I honestly don't care much either way, but I think it'd make sense if one of the other dragons appeared.