I don't know if you've noticed it but there are a lot of in-game references to time and the Bronze Dragonflight in MoP. What follows is pure speculation on my part (you've been warned).
Next expansion: World of Warcraft: Sands of Darkness.
Pre-expansion patch (5.5) (scenario)
Garrosh Hellscream is being judged by the Shado-Pan and a delegation of both the Horde and the Alliance (the players are amongst them). Found guilty of the destruction of the Vale, the genocide of trolls and randomly killing bunnies, he is about to be executed by Taran Zhu, when members of the Infinite Dragonflight appear and try to stop the execution. The players kill them and then enter Alextrasza, Ysera and Kalecgos (either warned by Jaina or Thrall). We learn that following Soridormi's death Nozdormu disappeared and the Bronze Dragonflight has been reporting strange occurences: time rifts opening here and there.
On her way out, Ysera congratulates Thrall on finally being able to close the Maelstrom.
Pre-expansion event: Through Azeroth, rifts open. In order to close them, heroes have to relieve part of the history of the place.
6.0: Pursuit of Time.:
After the Pandaren Campaign, the Horde and the Alliance try to maintain their uneasy peace. Heroes of the Alliance are called to Dalaran (which has finally arrived at the ruins of Theramore) by Jaina and those of the Horde join Thrall, the new orcish leader at Orgrimmar. There they meet Kalecgos (A) and Ysera (H) who tell them of a landmass that just appeared where the Maelstrom used to be. It is not (another) previously unknown island this time but a vision of the past: the pre-sundering Azeroth (or part of it). The appearance is probably linked to the missing Timelord and the Aspects want the mortal races to investigate.
A new Island divided into 5 zones + Azshara's city as a capital. The timeline is around the War of the Ancients (before or during I don't know) and player progresses from 90 to 100 through the zone while slowly unraveling the mysteries of the time anomaly and its link to Nozdormu.
Three New raids: After the players discover that Nozdormu has been captured by the Infinite Dragonflight, their leader Murozond comes from the future to try to corrupt them.
Echoes of Warcraft
Trapped in their past, the heroes has to defeat corrupted versions of the villains they defeated.
Masters of Outland (Vash'jir, Kael, Illidan) - Heralds of the Legion (Kil'jaeden, Archimonde) - Trial of the Old Gods (C'thun, Yogg'saron, Shas) - Frozen Throne (Lich King) - Madness of the Aspects (Malygos - Deathwing)
Corrupted by Murozond at the end of the Echoes of Warcraft, the players are tasked to collapse three timeways.
Each boss will function as a kind of scenario with steps. I don't know which timeline Murozond would want to be rewritten but the lore is full of stories that could be used).
The fourth boss is a Norushen-type of fight: players try to regain control and purify themselves of Murozond corruption. The raid faces a mirrored version itself (if technology allows it, the opposite team will use corrupted models of the players and their names). Once the corruption is beaten, the player return to the present while the Bronze Dragonflight go back to repair the timeways (see: scenarios)
The heroes assisted by the Aspects defeat various Infinite Dragonflight bosses in order to reach Nozdormu. It's a traditionnal raid culminating in defeating Murozond and freeing Nozdormu.
3 Scenarios: The three timeways we helped collapse seen from the other side.
6.1: Random Troll Patch
Well, it seems to be a tradition, each expansion its troll patch. So I let you imagine another way to kick some Zandalari ass (I personnaly don't like the troll patches).
6.2: The Queen of the Deep
Nozdormu, still reeling from Soridormi's death, can't make the Ancient Azeroth continent disappear back to its time, and a Naga invasion on its shores doesn't help. The proud Queen Azshara (the naga one, not her past self) sees the island as her rightfull queendom and uses her full might to attack. A quick questline and some individual scenarios phase the players in modified by conquest zones (phasing). After they are done, the player has access to a new raid:
Abyssal Maw (2.0)
The raid begins in Ancient Azeroth where the players have to regain control of the capital city by defeating the naga forces there. When it is done, the players follow the fleeing Queen to the depth of the ocean where they fight her and save the missing Neptulon (cataclysm loose end tied, you're welcome).
6.3: The Descent
A quest-line showing how the interaction between past-present-future affect an already weak Nozdormu, slowly driving him mad. The quest-line ends with an altercation between Nozdormu and the Aspects. He leaves and Ysera is afraid that her brother is on the path to become Murozond.
6.4: Fall of the Timelord
After his outburst at the council of the Aspects, Nozdormu has sealed off the Cavern of Time and the Bronze Dragonflight with him. Through the Dream, Ysera has managed to discover that Nozdormu's fate has not yet happened and send the heroes to try and save him.
Cavern of Time
Upon their arrival, the heroes discover that the Hourglass isn't pure anymore, black and white sand grains have started to appear. It is a race against time: every 15 minutes a fraction of the sand darkens, empowering the final boss (adding a few seconds to the dark phases). When defeating a boss, one player (three in 25M) gains a buff that allows him to clean a part of the Hourglass, the buff is more powerfull if said player(s) respected the mechanics of the boss. It makes for a very dynamic raiding experience, where the sense of urgency is real. (While the raid is off, the timer stops of course).
Final boss: Nozdormu. He awaits under the Hourglass, at the resting place of his beloved Soridormi. He is friendly to the players at the start. He tells them that he is preparing to go back in time to destroy Soridormi's killer before her death. Preventing that event will erase a timeline created because of him and his influence over time. The other Aspects try to stop him because what happened happened and he is the Guardian of time, not its maker. He traps them in a storm of sand. He starts channeling a portal. The raid has to fight off waves of dragons half-corrupted. When the Nozdormu reaches 100 energy, a portal opens. A hero has to enter it while the rest of the group fights Nozdormu who enters in Dark Phase. He morphs into Murozond. The hero has to survive long enough for the raid to bring Murozond to 80%. The Dark Phase ends and so one.
At 10%, Nozdormu is too weak to maintain the sandstorm that kept the Aspects out of the fight and they stop him from morphing again. Cue the cinematic that shows how close the Aspects are, each sharing the pain from his/her past (except Kalec who's banging Jaina and feels pretty awesome about it) to help Nozdormu gain back control.
Voilà! What do you think of such an expansion? (Please don't turn this into a Dark Below, Me wantz Emerald Dream, or Where them demons at? Stay on topic .
(By-the-way, I'm French, so please don't mine my english and its faults)