Firstly this is pure speculation nothing more nothing less.
Ok with the decline in wow and mop coming to a close I think Blizzard will be pulling out some big names to revive alot of subs who may have left during the cata and mop era.
So lets discuss possible heros / leaders who could make a return and stir up some sh*t.
I mainly play alliance so I will start there.
Human - Said before Turalyon coming back with his battle hardened troops to reingite the war on Lorderon and perhaps dismiss Varian for letting the north fall into horde hands so easily.
Dwarf - With the earth finally at peace the diamond prison shattered around Magni and upon seeing his daughter and her son in ironforge a new dwarf storyline emerges.
Gnome - since he is in the brawlers guild I would love to see a stroyline were Millhouse Manastorm returns to the gnomes and trys to seperate the technology loving gnomes from the magic focued gnomes and start a civil war of sorts between the two divions.
Worgen - this one would be great Packleader Ivar Bloodfang and Lord Darius Crowley going against the wishes of King greymane begin a brutal assault on the forsaken in and around gilneas.
Night elf - Two first names same second name Captain Jarod Shadowsong and Warden Maiev Shadowsong. There is a huge chain quest that could be applied here and have the night elfs start to believe that Maiev's anger and zeal would/should be the way forward.
Draenei - Velen Nuff said.
Now I will not mention horde as that is not my strong suit so if anyone wants to take up the horde challenge by all means.