My tin-foil hat: There is a splinter faction within the Forsaken loyal to the Horde and are trying to find a cure for the undead disease. With the help of some of the Taurens and other Alchemist within the forsaken discover that their souls are truely missing and without them all undead will all eventually go insane. Meanwhile Calia SI:7 girl is killed in an attempt to help Gilneas by Cromush. The head of this splintered organization meets with Cromush in secret with his concerns over the queen and offers an alternative. Calie, risen as a Foresaken, using a new version of the plauge with the added acception of binding her soul to a locket she now wears.

Calia being the true heir to Lordaeron leads the revolution to take back undercity from Sylvannas. I'm sure in doing so they discover that she's been corrupted/or willing participant by something, old gods, burning legion, some new evil to take over the world. Either way she dies and Calia takes the throne.

She goes to the Alliance to see what she should do but soon realizes the plight of the Foresaken. They don't want her, she's an abomination and like the rest of the Foresken needs to be destoryed. Cruel events to those that have helped her try and save the forsaken then solidifies her alligence with the Horde (This could actually be something that shows her people are just needed to save the world AKA the Burning Leigon is attacking and she can't just let Alliance fear and mistrust stop her from saving the world she loves).