The new raid entrance in Vale of Eternal Blossoms made me wonder, if Siege of Orgrimmar after all won't completely be in/around Orgrimmar.
So, I took a look at the dungeon journal to maybe find some hints.

Now, this is what I found:

First, we will dive into the vaults below the Mogu'shan Palace, where we will be greeted by the corrupted waters of the Vale and three corrupted quest givers of the Golden Lotus.
Defeating them will clear the path toward the Sha of Pride, however, before we can engage him, Norushen (the titan construct from the scenario "Dark Heart of Pandaria") will intercept us, to cleanse us from our own corruption.
I would guess, that the corridors inside the raid will direct us toward the heartroom inside Dark Heart of Pandaria.

Now, the next boss is a proto-drake, which could mean, that we will enter an aircraft, that brings us to durotar, once the sha is defeated.

After landing in front of Orgrimmars gates, we will face a big, modified scorpion and two Kor'kron encounters. These are all outdoor fights and could take place in between the entrance and Grommash Hold.
After dealing with the vast army of loyal orcs, we push insode Grommash Hold. However, instead of Garrosh, we will face Malkorok, with Y'shaaris heart hanging above him.
The next encounter, Spoils of Pandaria has no readily available information, however Thok the Bloodthirsty has a phase called "A Cry in the Darkness", suggesting that we will enter the Cleft of Shadows.
Siegecrafter Blackfuses encounter suggests, that it takes place inside a room full of machinery. This could possibly be inside Ragefire Chasm.

The last two bosses could possibly hide behind the doors seen in "The Secrets of Ragefire".