But to have a new tribe blizzard will need a new mechanical because all tribes have his own. Murlocs boost themself, dragon if you have an other dragon in your hand, pirates weapon, etc.
I don't know enough WOW to know what are all possibilities but a think I would like to see is Naga. We don't have a lot of water minions in HS and that could make some great stuff. We could see something like: Water mark: Put a Water mark on a minion when hurting him. Drowning: if the attacked minion has a water mark, do something.
But I don't thing we will see a new tribe soon. For me 2017 will begin with a new keyword. I like to create cards so I have created like 30 news keywords and I think it's easier than create a tribes so chances to get a new keyword soon is more like 70% and new tribe 10% (20% nothing new :c )
I think that would be awesome, building your deck around a tribe like dragon or beast is great and makes people create specific decks for those tribes. and they all have their own unique things just like the guy over me said. dragons get effects if you hold dragons, pirates are all about weapons, beasts rely on eachother and murlocs buff eachother and are strong if all together