Actually, that's not a half-bad idea. Here's how I see that playing out (your mileage may vary):

-Sylvanas continues her conquest across the northern reaches of the Eastern Kingdoms, which includes creating new undead and deploying the plague. Eventually, this garners the attention of Tirion Fordring and the Argent Crusade.

-Incensed by the actions of what is essentially the new "Lich Queen," the Crusade attempts to fight against the Forsaken but is stymied because of the backing of the Horde.

-Tirion appeals to the Alliance for assistance. This could play out in a number of ways, but the end result: Gnomish fun-gineers deliver a doomsday device to the Undercity similar to the one that wiped out Gnomeregan. It is likely a secret attack made under cover of a joint Crusade/Alliance assault.

-Undercity is destroyed, or at least made uninhabitable, even for the undead. Most of the Forsaken are destroyed. The remnants of Forsaken forces are forced to retreat to the Scarlet Monastery, where the undead remnants of the Scarlet Crusade deal with the choice to either join the Forsaken or be destroyed.

-(Optional) Sylvanas is missing and presumed dead after the attack on Undercity. A new leader arises to lead the Forsaken as a faction: Battered, but loyal to the Horde, unlikely to pursue the "Death to the living" agenda.

Oh, this path of events is highly unlikely. It'd mean either heavy phasing in Tirisfal/Plaguelands or revamping the content that was just recently revamped. Scarlet Monastery would probably go away as a dungeon, and Blizzard seems reluctant to let go of the legacy dungeons.

Still, this seems like a feasible path. And it'd definitely rock the boat for some factions and lore figures that need it.