I'm honestly kinda surprised they haven't already done that shortly after Pandaria's launch.
They clearly put a lot of love into Pet Battles in WoW itself, and can easily stand as its own game. Hell, it's one of the few things that actually could get me back to WoW again if they made it correctly.
I mean, it's a no-brainer to make a game that cross-connects to your WoW account to share pets, where the mobile game has their own set in addition to the base game - and you can bring said pets into WoW and visa versa?