Illidan. We are not prepared.
Arthas, reborn and wielding his old weapon and being infused by the light. They could easily add some quests where we hear of a scourge disturbance in in Northrend and when we arrive we find a scourge camp completely destroyed. After more quests/investigating we find that they brought back an extremely powerful being from the dead and later we get saved by Arthas
Let him be redeemed and wielding the power of light and shadow. Let some naaru sacrifice itself to make him a living, breathing human again and he can go back to Jaina. Kalecgos can go back to the one dragon he was suppose to be mated to (stupid Kalecgos siding with the Alliance).
If he cannot be brought back to life, then I would instead have Grom briefly return to see what Garrosh has done. Confront and scold him for his wrong doings, for the crimes he has committed. Let Grom inspire Garrosh and help teach him what it means to be a true orc. Maybe Garrosh could potentially feel guilt and give his life by doing something heroic. Something like saving Anduin from an attack from a top general of the Legion, having Baine and Thrall forgive him for his actions and letting Varian finally seek peace with Horde.
those two would be my votes
Although something tells me he isn't dead (turned into some kind of legion puppet, raid boss next expansion? likely.)
Deathwing! I want everyone to do it again
I want my Azjol'nerub raid with him at the end, old gods are hard to kill, right? He was spread all over Northrend, Ulduar was merely a setback etc etc.
This needs some thinking. I can't bring back someone too old I mean although Cairne was awesome, he was old and not that much left to live for him so no. Same for Terenas.
I think I'd bring back Uther, the Alliance needs a powerful figurehead and who's better to fight the Horde and the Forsaken then Uther?
Kel'Thuzad, we didnt destroy his phylactery, make him kill Sylvanas and lead Forsaken into new age.
Socks. Such a selfless lore character.
Bacon is the thing pigs give you when you're good.
Broxxigar the Red father of Varok Saurfaug as new warchief of the horde.
Or Cairne I miss him.
The Lich King, son of durotar father of broxigar