So in a dev interview, when asked about their favourite feature of Legion, the devs "couldn't talk about it". The first and second rule of Fight Club state that you can't talk about it, so this can be considered definitive proof of Fight Club coming to WoW! Yay!
I am super curious about how they will implement it. Brawler's guild and PVP are alright, but it's not quite the same as unarmed bashing each others faces in.