Brock's not coming back, he's getting a single episode at the end of BW to meet Cilan. He's not in the XY series and truthfully nobody should want him back after how poor he was in DP.
Brock, much like Misty, had the misfortune of being poorly received in their last respective sagas, and then replaced with characters who became just as popular or moreso than they did. So the writers decided to never bring them back to the main cast. This is why Misty has not had an episode return since 2005, and probably never will again.
This will probably be Brock's final appearance in the anime before the writers allow him to fade away into oblivion along with Misty.