Next expansion storyline revolves around Old Gods and Alliance vs Horde massive war (Every time when there OG around Alliance and Horde at each others throats - Vanilla/WoTLK/MoP).
Under leadership of Tyralyon Alliance gathering forces at Thandol Span to invade Arathi and capture Stormgarde. With established base there they plan to push farther into North to reclaim Lordaeron and Gilneas back. Danath Trollbane/Genn Greymane/Muradin assisting Tyralyon as field commanders of their united forces.
At same time Alleria trying to convince Blood Elves to change sides in upcoming conflict. Lothremar refuse. Forsaken attacking Aerie Peak in Hinterlands completely destroying it and planing to flank Tyralyon forces at Thoradin's Wall. While main Horde forces gathering over other side of the wall in Hillsbrad.
PC as leader of their Class Order sent to A/H to stop incoming war. Don't know how its goes but we ends up fighting alongside our faction at Thoradin's Wall. In middle of battle PC forced to go back in Class Order were we find what our CO in middle of civil war (Alliance members vs Horde members). Mentor figure (idk what it means probably NPC who helped us in Legion CO like Darion in DK storyline) disbands CO and sending us to Stormwind/Ogri where ship from Kul Tiras/Zandalar arrives. On the ship we found everyone dead except one member of the crew who completely mad.
Expansion's storyline divided on two parts:
1 PvE where we go to new lands and fight OG forces
2 PvP Lordaeron War
If i get it right start of Lordaeron conflict is prepatch that ends up with inspecting arrived ship. When expansion hits we go explore KT and near isles / progressing Lordaeron conflict.
Some interesting facts:
1 - Naga will not be in the start of expansion but rather they will appear as story of expansion progress.
2 - Kul Tiras will be more like Innsmouth: its citizens will be strange and weird keeping some dark (i would say tentacle) secret.
3 - Jaina is missing when we arrive to new zones. (Where is my waifu!?)
4 - We not heard last of Vol'jin!
5 - Infinity Dragonflight will be first main enemy we meet in new expansion (Murozond raid?) only then OG presence is revealed (And maybe Naga?).
6 - C'thun/Yogg-Saron will make reappearance.
7 - Genn vs Sylvanas story ends up in Lordaeron War. Who wins and who dies?!
8 - There hints what Alliance and Horde lose one of their cities (capitals? my bet on Darnassus) in EK/Kalimdor when N'zoth and Naga reveal themselves.
9 - In second part of story we go underwater and into shadowlands (Realm of Shadows?) to take fight with old gods in theirs domain.
10 - Lordaeron War is HUGE pvp event during whole timespan of expansion. Prepare to conquest/defend/capture/siege! (Improved Karasang?)