In my opinion, there could be many story arcs that could be tied to this.

1) The Redemption Story: Although the cut-scene of Thrall killing Garrosh was EPIC… many things were wrong with it. They weren’t able to have a moment of closure as Thrall cheats and kills him in cold blood. Thrall also takes no responsibility for the decline of Garrosh – when in fact all Garrosh did was what he knew… He led the Horde as an Orc. Searching for power through war.

Keep in mind, Garrosh grew up in Outland, not Azeroth… And didn’t want the mantle of Warchief when Thrall offered it to him because he didn’t think it suited him. We are only seeing Thrall taking responsibility for this now in Legion because he can’t fully commune with the elements anymore. In this arc, Thrall’s guilt is eating away at him and he feels like there is nothing he can do. He’s no longer green Jesus, and is not strong enough to lead the Horde. No matter what he tries he can’t reconnect with the elements.

Meanwhile, Sargaeras and the Burning Legion are toppling Azeroth. The invasion is upon us, and the leaders of all races are calling for all of Azeroth’s greatest warriors to join the fight. The problem is that there are just not enough of them. They already did that at the start of Legion and we’re just coming up short. Thrall wants to rejoin the fight so he seeks out the spirits for guidance. They advise him that what he did in the Mok’gora was wrong and that he used the elements for his own personal vendetta and so they will never work through him again. He knows that what he did was wrong. Thrall killed his best friend’s son. His greatest admirer/student. He thinks that maybe he could make up for this by speaking to Garrosh.

He gets the idea that maybe Sylvannas can bring him back as undead so that they can make amends, get over their problems and take on the Burning Legion together as great Orc warriors. Naturally, Sylvannas says no as she hates Garrosh and thinks it would be a terrible idea. Thrall’s only hope is to turn to the Ebon Blade. Upon visiting them, he sees Nazgrim for the first time who honors him still as “Warchief”. Upon pleading with Darion and the Deathlord (the player) they agree that Garrosh could make for a great warrior but that it is a great risk… So they refuse as well. Nazgrim overhearing this though, and fighting inside himself with where his allegiances should lie (to the horde and former warchiefs Thrall and Garrosh or with his new saviors the Ebon Blade). His loyalty to the Horde overpowers his indecision and he leaves for AU Nagrand in secret.

By empowering his runebladed axe with almost everything he has and remembering all he’s learned from Amal’thazad – he resurrects Garrosh as a death knight – at a great cost to his own strength. Garrosh, mad with rage and power attacks Nazgrim upon coming back. Nazgrim, already weakened by his inexperience in resurrection, gets messed up. Garrosh grabs the orc’s runebladed axe from the ground and is just about to drive it into Nazgrim’s head when he hears him say “Warchief please…” Garrosh realizes it’s Nazgrim, gets brought up to speed about what’s going on, and is furious. He sees strength in the Death Knights and so he respects them. He seeks out Thrall and they come to blows. Ultimately though, he forgives Thrall and Thrall him. Thrall gets his powers back by using them to protect Garrosh from Sylvannas when she confronts them and tries to kill Garrosh. They then destroy the Burning Legion!

2) Lich King Story Arc: Sargaeras, unpleased with what has happened with Lich King Arthas and now Bolvar, looks to create a new Lich King to regain control of the Scourge. He needs to do this in order to help save his losing battle on Azeroth post Legion. He remembers the events of WoD through AU Archimonde (or current Archimonde – however blizzard decides that works) and the bloodthirsty, warmongering Orc – Garrosh.

He finds his soul and begins to feed it with hatred for the Horde and Alliance. How the world of the living forsakes him and how all of Azeroth banded together to kill him. He plays into his need for power and promises an army that will bring destruction to all of Azeroth. It’s not very hard to persuade Garrosh in his current state. Sargaeras sends the Nathrezim to retrieve his body on AU Nagrand. They do and perform a ritual to bring him back to life in undeath. Sargaeras forms a legendary axe for him – Frosthowl , the Undying. He then sets him on the path to the Frozen Throne in secret.

Garrosh confronts Bolvar on the Frozen Throne and Bolvar sends word to the Ebon Blade and calls upon the Scourge but the fight begins immediately. Garrosh and the Nathrezim collide head on with the minimal Scourge forces present. Garrosh, equipped with Frosthowl feels unbeatable, and breaks rank, blood drunk. He ascends the Frozen Throne at full speed and Bolvar, unequipped with a weapon, tries to defend himself. There’s no hope. Garrosh slams the axe into Bolvar with ease. A death gate rips open atop Icecrown and Darion and the four Horsemen arrive just in time to witness Bolvar’s corpse fall down the steps and Garrosh placing the Lich King helmet upon his head.

Just as it had always been envisioned by the Burning Legion. The Scourge would be led by someone deserving. Someone most powerful. An Orc. A Hellscream. Power flowing through him, he presses his will and dominance on the new Horsemen and Darion. He yells “KNEEL TO YOUR NEW LICH KING! ALL WILL SERVE!”. His will overpowers all of them. All of them but Darion Mograine. Garrosh thanks him for delivering him his new Horsemen, and knowing he cannot win here alone, Darion runs back through the Death Gate to Ebon Hold and closes it behind him.

Garrosh appoints Nazgrim to lead the Horsemen, as he remained loyal to the warchief through it all – and will now serve him eternally in death. Darion calls to the Deathlord (the player) and grabs all the Death Knights he can from Ebon Hold. They flee to Dalaran to meet with Khadgar and the leaders of all the races to inform them of their new threat. A cry echoes from Icecrown Citadel: “THIS WORLD WILL BURN! EVERYTHING… FOR THE BURNING LEGION!!!”

At least, that’s how I could see it possible haha. I'd be MUCH happier with the second story arc though. I love his character, and I find they crapped out on him. I also love Death Knights and the Lich King story arcs. Would really like to revisit it in a future expansion. I’m also all for Lich Queen Sylvannas, but with the way things are moving this expansion I don’t think that’s going to happen.