Yes, battlegrounds will no longer be locked to battlegroups in the coming expansion. The pool of players to pull from is going to grow dramatically with this change, which can only be a good thing. It also has interesting implications for PvE content. Cross-realm battlegrounds came long before cross-realm 5-mans via the dungeon finder, so cross-battlegroup battlegrounds could potentially suggest that the dungeon finder will pull from the entire region eventually as well. With the Battle.net revamp and the incoming Real ID features, what realm we play on may become less and less relevant for anything but the guild we're in and which faction holds Tol Barad.
[via World of Raids]