"Rise of Lordaeron"
I would have Lilian Voss make a return, she would re-appear with a new faction of scourge and bring battle across the ruins of Lordaeron against the forsaken. At the same time boats from Kul'Tiras appear at Stormwind bringing tales of undead rising and taking their island from them. After it is determined these undead are coming from Kul'Tiras by both the Forsaken and the Alliance we ship off to the tropical islands that Jaina Hails from....
When we arrive at Kul'Tiras we have to solve the mystery of why and how these forsaken have risen. As we quest through the areas of this land we find that Lillian Voss has stolen some of the original plague and used it to create her own scourge. The first Two raids involve going in and first stopping her production of undead players, and then facing down and stopping Lillian Voss.
When we beat (not kill) Lillian however, it is discovered that she had created this scourge in response to her discovery that Sylvanas has been experimenting and creating her own version of the scourge which she intends to use to wipe out all the living Horde and Alliance alike. She plans on using the current forsaken as fodder as she marches her new and improved people to war against the living. While Bolvar has been asleep fighting the lich king's influence, she has even stolen several undead from Northrend with the help of the Val'kyre. The second raid involves us going in and stopping her production of the "super" undead, with Lillian Voss helping lead the way. The end boss will be Koltira Deathweaver who had been re-purposed by Sylvanas to be the general of her new army.
No new race, but the new class "Nightstalker" will be added. Nightstalkers will be a mail wearing class that shifts from ranged to melee combat depending on the spec. Nightstalkers are specifically trained in using magic to fight off undead and demons. However, they use necromatic magics instead of the light to fight their prey. (Ok, I want Demon Hunters, but with a name that includes the undead etc.. as one of their victims.)
The last bit, and raid will be around us fighting Sylvanas, who will be the boss midway through the raid. As we fight her, we will find the burning legion has been controlling her movements. As she lies dying Vellen uses his magic to heal her, but ends up resurrecting her back into a living High elf/Blood elf. This breaks her connection with the new scourge army, and she is taken away by her sister Vereesa, and we head on to fight the burning legion and remove the rest of the new scourge from existance.
This of course will lead into a burning legion expansion, and will also set up Sylvanas' redemption story. It will also leave the Forsaken's future into question. Will Sylvanas return to lead them even though she is now living? Will Lillian Voss be the next dark lady? Having to fight alongside the horde, and the Forsaken do the Alliance now have to recognize the Forsaken's right to Lordaeran as their home?
I know this pitch idea is not perfect, and I am pitching this as an Alliance player who has always loved Sylanas' story and have hoped for since Wrath to see her story progress. But here is the real question.
What would you pitch for a new expansion? What would you like to see that hasn't been shown before? What new lands/planets/dimensions would you like to explore and why?