Regarding the Arathi and their usage in a future plot...
I've started to feel it'd be best to use them heavily in Midnight, although never granting us a full view of the Empire. After all of the talk of Renilash, I cannot imagine a world in which at the very least the Hallowfall Expedition is journeying to Quel'thalas with us. We should be able to see a portion of their strength and further develop the culture, fueling both the mystery surrounding them and their stance as a culture that will have a lasting impact on the franchise.
I think one of the best things about them is that there are truly so many directions to take with just the Hallowfall Expedition. They can continue trying to reestablish the connection with the Empire, they can shack up in Stormwind, repopulate the Arathi Highlands, build a stronghold in the former Plaguelands, convert the Scarlets to their cause, gain interest in the Sunwell, learn of the Naaru, etc.
There's a lot that can be done and I think a slow burn on them with heavy involvement before we see their full Empire will really make it worth it.
That being said, 14.0 Avaloren