We know an Old God offered the Highborne power in exchange for their loyalty - that's how the nagas were born. We dont see much action from them so we assume the Old God is waiting for a better moment to begin his assault. So , what better than all of Azeroth's heroes travelling 30 years in the past in a different timeline?
The Old God who is controlling the naga could be someone who was defeated - C'thun , Yogg'saron or Y'sharj (whatever was his darn name) , but because the naga didnt assist him while he was getting owned , we can assume they are loyal to somebody else.
It's said there is a figure behind Garrosh so why not an old god? (another one , it's possible).
Also , someone or something has to give Garrosh the power to travel to a different timeline 30 years in the past. It might be Karios , since a "friend" helps Garrosh escape and he was mentioned a couple at Blizzcon , but i doubt he has enough power to do this.
And as I said in the previous spec thread , the final boss is NOT an orc , so an old god or atleast Azshara is possible.
P.S. same quote for the grammar