The recent purge of Dalaran allowed us to see players really going at it supporting their factions fortunately enough it gives us a good understanding of what players really think is wrong and right with their factions, well made criticism than just venting that the horde/alliance is winning everything.
As a Horde player I like the idea of the Horde being the underdog (like it was intended) I like to see it rise from turmoil however during Cata and MoP the Horde seems to be a juggernaut (Not a bad thing necessarily) but compared to the Alliance (Most of the time played as the weak faction due to the number of zones and quests the story revolves around them losing) it creates the obvious faction discussion we see every time, fortunately with the knowledge of a siege of Orgrimmar I had the chance to imagine a fun scenario you might consider worth the read.
First and foremost Horde players still have Orgrimmar as a capital.
After Garrosh is dealt with the Alliance again sees the threat of bad orc leadership continues to unleash in the world, Alliance leaders (Your choice of whom might be my take would be:Tyrande,The council of the three hammers and Greymane) decide to occupy Orgrimmar (Not creating the second war camps) installing a "War council" of Horde representatives with an Alliance representative in charge of Horde leadership.
Changes to Orgrimmar would be the prescence of heavy alliance military (Like Kor'Kron in the Undercity) to act as guards and we get to see in progressive aggression towards orcs and other horde members, to kinda hint at a future liberation of the city.
Horde leaders are of course in uproar at the decision however after spending their resources in the siege, the war and overall low population of Horde races they have their hands tied some however see it as an opportunity to further their goals.
- Alliance guards to replace most Orgrimmar guards, these guards are not entirely friendly to players however they have that red/orange name meaning they are not friendly.
- Parts of Orgrimmar are destroyed after the battle and we see horde members rebuilding their city, maybe creating different districts where different horde races live.
- Quests after the siege feature players helping guerrilla liberation forces and helping common people improve their quality of living after the occupation (Eliminating evil guards/Commanders etc)
- Big Night Elf and Gilnean prescence in Orgrimmar and other contested zones in a sort of retaliation
- Draenie are in a position of turning the other cheek towards the orcs accusing brutality and bad treatment as the cause of a future insurrection
The obvious direction this is going is to choose a new "Thrall" is you want to call it that way that liberates the horde once again , the liberation of Orgrimmar can be the center piece of the next expansion where the big war is supposed to occur.
Edit: Changed Hostile to not friendly