BGs are content, CRBGs are a mechanic of that content. Just my take, anyway.
However, because it’s a hybrid, I think they will make a special rule:
Pre-level 60 brackets will be cross-realm, and level 60 brackets will be realm-only. This helps deal with the reality of having several active brackets without taking away from the primary realm feeling at level cap.
It’s just my guess, and it’s far from perfect…perhaps too convoluted for Blizzard to actually do.
Being realistic, I think they WANT to keep cross-realm out of BGs for as long as possible, but are yet to make a concrete decision. As I’d like to repeat, I think CRBGS is a unique part of the game that doesn’t fit squarely into anything they’ve talked about…but I could be wrong.
There are implications everywhere, however.