My speculation is mainly on the bosses and events, nothing in a particular order. Feel free to post all speculation you've thought.
Bosses
* Hakkar the Houndmaster - Aided by Malfurion, heroes will destroy Hakkar the Houndmaster.
* Mannoroth - Aided by Cenarius, heroes will defeat Mannoroth and push him back
* Archimonde - Aided by Malorne who dies fighting Archimonde, who vanishes shortly afterwards.
* Xavius - Aided by Malfurion, heroes will fight and destroy the future Nightmare Lord.
* An Infinite Dragon - Attempting to kill Deathwing, he must be stopped by the players.
* Deathwing - Using the Demon Soul, he is unstoppable. Players are by accident teleported to him, and must survive long enough to escape.
*Summary*
I prefer to think of this raid as being the entire war stretched out, with the heroes witnessing only the most important parts of it. Most bosses are in their own area, these areas are surrounded by fog and adventurers cannot escape. When the corresponding boss is defeated, a portal appears allowing the heroes to travel on. The demons will be fighting various allies at any one time, giving a more "warlike" feeling.
Hakkar the Houndmaster is located in his own location, in a circle surrounded by ferocious felhounds.
Mannoroth is in his own area, behind ranks of the Burning Legion.
Archimonde is also in his own area, confronting Malorne.
Xavius and the Infinite Dragon share an area. When Xavius is killed, the Infinite Dragon appears, and fights the heroes so nothing could prevent him from killing Deathwing.
When the Infinite Dragon is killed, he teleports the heroes to Deathwing so they could be killed. The Heroes must survive long enough for the Bronze Dragons to teleport them out.
Conclusion
I mainly wanted this "raid" to bring back an enemy from the Burning Crusade, yet introduce players to Deathwing, if in another time. To see how he has changed, from Neltharian to Deathwing.
Mannoroth deserves a place in the Caverns of Time, Archimonde needs another raid to be in, and Hakkar the Houndmaster has never been seen before. Xavius will never likely be seen in WoW since he's a fucking tree now, so this raid would be perfect for him.
Also we need an Infinite Dragon screwing around, because they are cool.
That is all folks. I'd like to see your ideas for this as well ; )