I am not sure MOBA Battlegrounds would be a reason to come back to WoW. There are plenty of Dota/Moba style games out there that don't require the same amount of time sink and sub.
It might however be cool for the people that currently enjoy the game. Blizzard over all has habit of doing things like this before like having a chess game, plants vs zombies, and other little mini-games.
I am not exactly sure how they could handle the Moba progression if they allow max level characters and epic pve/pvp items. They would have to make the gear and characters you use separate from the main game. If not you might as well get rid of the death penalty and feeding.