I started writing this for the "possible bosses of 5.4" but then decided to run with it as I had a lot of fun writing it out. This is how I see Siege of Orgrimmar unfolding:
We will begin with a siege of the front gates, heavily fortified and blockaded. We will use machines sold to us by Gazlowe the Honourable Goblin to mow down hundreds of Gary Loyalists. Upon reaching the gate, a giant Goblin Tank will come forth. A vehicle-combat encounter will ensue and after knocking it back into the gate, the Tank will blow up and take the gate with it.
We enter Orgrimmar and come to a fork. To our right is Warlord Zaela riding a tamed protodrake. We will have to fight both her, the drake and then both of them in tandem using conviniently placed harpoon launchers to bring the dragon down to us. To our left is the forge where a Dark Shaman Molten Behemoth Construct assults us and occasionally throws players into the molten steel. Blocking the way forward is Tradeprince Gallywix in a giant Mech. He's there to protect his investment! Upon defeating him we can progress past the Second barricade.
We're presented with another fork. To our right is a door with a large box-shaped groove and several large padlocks that we cannot open. To the left is the new Blademaster Training Grounds. Here, we fight council of three Blademasters all highly skilled in using pure steel weapons. We can skip them and head straight to Ragefire Chasm which is now much more chasmy. The entrance is guarded by a giant stone construct that came straight from Gary's imagination using stolen Titan technology. We have to fight individual limbs of the giant construct because it's just too huge.
We pass him and move into the chasm. Each of Garrosh's greatest War Veterans are in a different section of the Ragefire. Guarding the walkway connecting each section is Shokia equipped with her handy rifle and accompanied by her many pets: Wyvern, Wolf, Boar and Tiger. To the North is Kiryn and she's got some hulking Forsaken Abominations guarding her! One of them is called Koltira. Sad times. To the East is Rivett and he's been working with the Dark Shaman to develop a range of ice-based weapons determined to really make their enemies slip up! Finally, to the West, is Riko! Riko isn't really sure why he has to fight his friend but what he does know is that he has been given access to more explosive, rockets and lasers than he can hold and he is very excited by this. It's nothing personal. Riko just like boom.
We progress deep into the depths of Ragefire. General Nazgrim confronts us and says that he's sad that he has to engage his blood sworn allies but that we cannot be forgiven for our betrayal of the Warchief. He says that even though Garrosh's methods may not be ideal he does truly believe that Garrosh has the best interests ofthe Horde at his heart. Garrosh reveals himself and "rewards" Nazgrim for his loyalty by forcing the power of the Sha upon him... Twisting his body beyond recognition.
At the end of the fight, Nazgrim's task force all come forwards and try to calm him down. They all realised, with the way Garrosh used Nazgrim, that they're not fighting for the right side and join us in the march upon Garrosh. Thanks to the courage, love, hope, confidence, peace and happiness with which they confronted Nazgrim he is able to bring the power of the Sha under control and uses it to clear the way to Garrosh for us. His final words are those of pride in his people, his team and himself for overcoming the corruption of the Sha. He then disipates into a veil of Mist from which Garrosh runs out screaming and swinging for us all.
The fight with Garrosh begins with him fighting us directly. It's a seven mini-phase fight where he uses each of the negative emotions of the Sha to empower him but making it very clear that he has mastery over them and knows exactly what he is doing and has known all along. As we begin to overpower him he unleashes his secret weapon: The Dark Heart of Pandaria! Summoing forth an Avatar of Yshaalrjaljrjgh and this plunges us into dark visions from which we must escape.
Upon defeating Garrosh on Heroic and returning the Dark Heart to it's Titanic Containment Unit and taking a key from each of the Heroic General encounters we are able to unlock that door that we passed. Within is the Sha of Pride! It's a secret, Heroic-Only fight where we are given a glimpse of what could be the downfall of the Alliance and Horde and we resolve to overcome it with humility and respect and by embracing our fellow man we are able to purge ourselves of pride. Just in time for a single Infernal to rain from the sky killing Vol'jin on impact and getting one-shot by Lor'Themar. Wrathion arrives and congratulates Lor'Themar on saving Azeroth from the Demon threat he foresaw and promotes him to Warchief but warns him that Naga are surging forwards from the Ocean.