The way I envision it:
- tier 17: Azshara begins an assault on the surface world, bringing her minions, monsters from the deep and Old God powers to bear, suffers a setback as the surface races strike back (perhaps in her own city of Nazjatar), then flees/is recalled by her master
- tier 18: Nerubians call for aid, offering ancient knowledge in exchange - knowledge necessary to fight N'Zoth and successfuly defeat him; Horde and Alliance soldiers fight in A-N to free the city from Faceless Ones, scourge remnants, undead Nerubians and whatever else crawls out from the darkness
- tier 19: we bring the fight to N'Zoth's domain, descending through the Emerald Dream into Nya'lotha, facing his minions, Azshara and finally the Old God itself; lore unearthed from A-N is used to defeat/subdue/permanently banish N'Zoth
Azshara, before going down for the second, and final, time, mocks the attacking raiders about their fate being sealed anyway, as her first master is coming. Ding, 6.x ends, 7.0 begins, Legion says hello.