But, since WoW runs on Rule of Cool, I suppose the war effort should as well.
So... something like this should have enough of an epic flair and bite:
Blockade and Rebellion
The Alliance (like the Brits and historic Allies are so fond of in real life) conducts an 'Operation Anaconda' blockade around Durotar. While the fleet and surface raiders sail in, the real punch is going to be conducted by gnomish submarine wolfpacks under the command of a named Gnomish Admiral. Alliance heroes launch from a submarine's hangar to identify Horde convoys inbound to Durotar with laden with foreign spoils. Gnomish submarines then destroy them, while Alliance pilots provide air cover with the help of a flying airship. Their scenario is then to help secure one of the Horde ships deemed too valuable to sink, and recover the artifacts onboard. They insert either via air-drop or, more interestingly, from submersible.
The Horde, concurrent with the sea battles off the coast, is assisting the war effort though the time honored contribution of all resistance movements.
Horde resistance fighters are tasked with several assassination missions of increasing difficulty to help cripple the Loyalist war effort. We've already seen a few Shado-pan quests like this, and I actually enjoyed them (though things could get annoying with the pet running around, dumbass demon). In addition to setting up ambushes (maybe even one or two based on history, like how Operation Anthropoid [http://en.wikipedia.org/wiki/Operation_Anthropoid] famously dealt with the Hangman of Prague, Reinhard Heydrich)
The scenario for Horde in this phase is a raid on their base of operations in or near Orgrimmar! Loyalist Horde forces storm the hideout where Vol'jin and the rest have set up shop, and Horde heroes fight a rear guard as the leaders of the rebellion make their escape into the countryside. After a fierce battle to escape, possibly including the unexpected commandeering of a machine gun, Vol'jin vows that the rebellion has only just begun, and they set up a new redoubt in the hills, to secure Durotar and tighten the noose around Orgrimmar!
The Beaches and the Hills
With a blockade over sea and land now in place, strangling the Loyalist Horde and cutting them off from further reinforcement, the Alliance makes a landing, "Saving Private Ryan" Normandy style. Alliance ships bombard the coast in a sustained barrage and Alliance heroes load up to lead squads of men ashore (again, a mechanic we've seen before, reused). Garrosh launches suicidal bat-flyers or proto-drake flying orcs (Dragonmaw, then) to try and stop the invasion. While the air war is going on (much like around Wyrmrest), Alliance forces establish their bloody beachhead against Loyalist Horde resistance. It culminates in the construction of an armed Command and Control HQ and the landing of various Alliance leaders.
This sort of thing calls for crack shock troops, so why not have it lead by worgen detachments of special forces, lead by the Alliance hero? Storm the beaches and secure the LZ!
(prob no added scenarios in this phase)
Horde, meanwhile, have intercepted Loyalist communications about the invasion. Vol'jin is already in contact with Alliance leaders, he reveals, and they have been covertly supplying the rebellion with gold, supplies, explosives, etc. The rebellion's job is twofold now!
First, they have uncovered that one wing of the Horde forces tasked to suicidally defend the beaches is likely to defect if given the chance. It is composed of trolls and tauren, and other political undesirables, forced onto the front lines to "die with honor" by orc overseers. "Dis ain't honor," Von'jin says, "It ain't honor when ya don't gotta choice." The Horde hero is tasked to support an uprising among this force! There are a number of cool ways this can be done, but if it is stealthy, then it could even end with the Horde hero masquerading as the commander of this lost legion, unknown to the Loyalists... something that can come in handy later, even.
Second, while the Alliance is taking the beaches, Vol'jin wants to launch an attack of his own while the Loyalist forces are already focused on the Alliance threat. He wants the Horde hero to break into a prison camp and free some of his fellow rebels who have been captured and tortured by the Loyalists. All this would take place in the same phase as the Alliance attack, filling the skies between Org and the beaches with air battles and other chaos, and the distant thunder of artillery. Horde players encounter Garrosh himself, ranting and raving as he leaves the prison camp. Once he is gone, the hero instigates the riot, saving important Horde leaders both from the guards and from other prisoners! They then escape into the hills, confident that the tide has turned...
War in the Streets
The battle is in the final phases. The Alliance and Horde players are finally brought together for the first time and introduced. Orgrimmar is surrounded and being pounded by artillery and airships. Garrosh, in the tradition of dictators, has pulled back his Personal Guard and elites in the Kor'kron to defend Orgrimmar to the death. He has also armed every man, woman and child and told the to "die honorably for the Horde. Lok'tar Ogar!" While he, naturally, retreats to his bunker where he's working on his superweapon.
Vol'jin describes intel and information that the rebels have on Garrosh's doomsday plans and convinces the Alliance leaders that they can not afford to simply starve and smash Orgrimmar into submission. Garrosh and his elite will never surrender and it will mean the death of thousands. Many in the Alliance are skeptical about rushing ahead with storming the city, rather than a besieging it, but Vol'jin and the others convince them, especially with the threat of a new Sha superweapon.
Once again, Alliance takes up the direct assault, by necessity, they have the heavy weapons.
Artillery is focused to make a breach in the wall, while ladders and steam-powered siege towers roll forward to take the walls and the high ground. An Alliance paratrooper division lands to secure the rear at Azshara and that is where the Alliance hero comes in.
Alliance heroes are grouped under the command of Shandaris Feathermoon, to form the spearhead with her sentinels (I'd personally like to throw in wardens, too, cause... wardens are badass !@#$%es). They drop into the Valley of Spirits and quickly take the gate there, to allow more night elf forces to storm inside. Garrosh unleashes some of his pet monsters gathered from previous expansions: Gronn and Ettin and Magnataur and other beasties. The nighr elves are more prepared this time, however, and they lead Garrosh's crack monsters into an ambush via the creative application of excessive explosives.
They then push forward, flanking the Loyalist Horde in the Valley of Strength and routing the enemy there, which had held the Alliance forces from the front in check. For the Alliance, the battle ends with the Loyalist Horde pushed back into the Valleys of Wisdom and Honor. Before the end, though, they witness Loyalist Horde killing their own retreating men, calling them cowards as they try and flee back into the rear half of the city.
Disgusted, Varian vows that the Horde leadership is corrupt but that the Horde regulars fought with honor, despite their leaders, not because of them. Something like that.
The Horde hero is also kept busy. While the Alliance attacks the front, Vol'jin wants to cut the head off the snake, right here and right now! He personally leads a foray of True Elites (including the player and named characters, basically like a small NPC raid or something) into the city, rappelling down like ff'in Spartacus or something down the side of Mount Vesuvius. Their mission is nothing less than to cut a bloody patch of chaos to Garrosh and mob the bastard.
Of course, plans rarely survive contact with the enemy, and Vol'jin is waylaid by Kor'kron death squads in the city, already amped up on Sha-juice. They're killing everyone, even their own people. Vol'jin makes the hard choice to delay going right for Garrosh to try and save the city they're fighting for. Maybe he thinks or says, "This ain't just a city, this city is the Horde; I ain't gonna watch cowards burn it down."
More Loyalist scum come out of the woodwork - a few turn coat at the sight of Vol'jin and his forces, but many more fight on! Vol'jin has to divide his forces between saving lives and ending them. Actually, I've neglected Lor'themar, I'm just now realizing. So maybe, for DRAMA, why not have Vol'jin get wounded during the intense fighting... and Lor'themar, no doubt a little tempted to consider what would happen if HE were the only man left standing in the Horde inner circle when the battle ends, still ultimately rallies around Vol'jin, gets him treated, and pushes forward.
Of course, by the time the Horde heroes reach Garrosh, informing him that his city has fallen and his loyalists are crushed, Garrosh dismisses the heroes and delves into his bunker/hold. The last of his forces are there, with his superweapon, which he assures us, is near completion.
The Alliance players and leaders finally catch up with the Horde ones, and the stage is set for the final showdown and raid.
Scenarios, I think, would be varied and tied to any number of objectives to seize in the city.
Truth be told, Bliz will probably make the whole siege a homogenous-set-up of a raid. I preferred to divide the raid into two parts: Horde scheme and Alliance scheme, that only truly intersect at the end.
That end, of course, would be a fairly small raid, sort of like Gruul's Lair. Maybe twice that size. It encompasses the surprisingly large (larger on the inside! Damn Time Lords) Hellscream Bunker. Where we face more of the abominations he wants to make the Horde into (an even more fallen-than-normal Warlock boss, the leadership of the Blackrock and Dragonmaw orcs, maybe an Illidari Council style fight, and other psychopaths who need to die). Horde and Alliance heroes both fight alongside Horde and Alliance NPCs here, and of course it ends with the death of Garrosh himself. I won't speculate on if someone else, a troll, say, or some guy with a funny chin, gets the final blow in.
The whole thing then culminates in a cutscene of the Horde and Alliance signing a treaty of some sort, ala Macarthur and Shigemitsu on the USS Missouri, with all the various leaders around. You don't even need to really go into just what the treaty involves, since I know Bliz hates the idea of revisiting or phasing "old content."
But we can at least use our imagination and hope it isn't status quo cata-bellum.
Also, cue some speeches from this or that. Whoever leads the Horde is the most important speech, since it will outline what the Horde 'means' to them and such. Notably absent, I suspect, would be words from the Forsaken, or perhaps some bitterness on their part. I excluded them from this, largely because I expect Sylvy would want to bide her time and marshal her forces, and not bleed them out in the streets of Orgrimmar.
And that, roughly, would be my epic take on the Siege of Orgrimmar.