Kaylriene has written his own speculation about the future of lore. I’ve started to answer in comments, then I thought it deserves a post of my own.
Here is what happens next:
1. Battle of Dazar’Alor leaves Kul Tiras and Zandalar with their fleets reduced to 5-10% of initiate might.
2. Azshara raises Nazjatar from the depths – there is no better timing to do that, and we have a chance to finish her. Upon dying, she says something like: Mwahaha, you’re late!
3. N’Zoth is actively corrupting Azeroth the Slumbering Titan, and we go to the heart of the world, Underwater / Black Empire, to stop him.
4. Our last playground is gonna be the said Underwater / Black Empire.
5. The final raid is tentacles, tentacles everywhere. The last boss of the expansion is ‘Corruption of Azeroth’ – we fight the tentacles or some sort of N’Zoth manifestation to stop the process.
6. It is important that we do not kill N’Zoth by ourselves. We will use the Heart of Azeroth as an ultimate remedy, unleashing all the collected power to clean the tentacles around her. This may either be a part of final cinematic or an encounter mechanic.
7. In the final cinematic, Azeroth is clean and awakened, and she kills N’Zoth with her bare hands – to show how powerful of a champion she is. This is gonna be cosmic and epic.
8. She says: ‘Thank you, my champions. Laters’.
9. She then leaves to see other titans, to gather strength, to learn and maybe even try to convince Sargeras back from Chaos. She says that ‘We will meet again to fight the Void’.
10. We are left on our own for now.
11. Details may vary, but my core idea is that we will finally see the awakening of a titan, and the planet will be released from the Old Gods grip forever.
12. The next expansion 10.0. is Death theme. We will get a revamped Northrend, with Bolvar/Taelia, Calia Menethil, Bwonsamdi, Sylvanas (which ran away, not dead), Helya (vrykul again) themes. The Checkhov’s Gun of loose massive undead forces must be solved once and for all before we leave to another planets to fight the Void Lords.