What i would like to see is:
You meet up with thrall, at the start of the raid.
He takes us to orgrimmar, with the intention to tell garrosh to stand down etc.
He will do some speech about how garrosh can redeem himself, by giving up his position as warchief and stop being so power greedy.
Garrosh will cry like a little bitch, send one of his cronies as a boss fight and he will run off, because he is shit scared of thrall.
Thrall will call him a coward, and rally members of the horde to search orgrimmar to find garrosh.
They could use a combination of different places that garrosh is hiding. For example in one raid reset, he is hiding at ragefire chasm, the next week he is at the rear gates.
They could give him different abilities/mechanics depending on where he is, and the raid would also recieve help from an npc faction that is currently fighting in that area.
Kind of like how yogg works with keepers, except you only ever get 1. One section of orgrimmar would have help from tauren, another from trolls etc.
Not sure how the alliance would play into it. But Horde and Alliance have always worked together to defeat a common enemy.