My speculation based on 4chan leak that was dropped during mid summer. It was first leak that mentioned next expansion as
"heavy female characters focused" before we even knew about Blizzcon schedule. My speculation is Blizzard will play out Pirates of Caribbean in next expansion. Here the quote of this leak:
Interesting isn't it? I found its also pretty logical - after death of admiral Daelin (W3 TFT) and Lieutenant Benedict (Vanilla) the highest ranking military member of Kul Tiras will be Lieutenant Alverod.
https://wow.gamepedia.com/Admiral_Proudmoore's_Orders
One more interesting fact i want to mention before starting my speculation - Taylor.
He helped Alliance heroes in Vashj'ir (Naga Zone) and was promoted to
Admiral in MoP. But then Blizzard killed him off and raised as
ghost in WoD. He was killed by this guy:
https://wow.gamepedia.com/Ephial
Do someone looking for and recruit undead admirals for their fleet?!
Speculation:
1 - Alverod was seduced by Azshara to create large fleet armada and join N'Zoth. Azshara will play out Calypso - both are called Sea Witches, both were beautiful but lost it because curse.
2 - Jaina arrives to Kul Tiras and finds it under Old Gods influence. She tries to save her homeland. It will explain her absence in Legion. Also remember those Kul Tiras marines in Legion? What if they were sent by Jaina to warn us but were intercepted by Naga?
3 - Alverod is "Davy Jones" of WoW and he commands his "Flying Dutchman". Heroes in order to find N'Zoth needs to reach other side of deep sea. The only way to do it - use local "Flying Dutchman".
4 - But our Davy Jones is immortal and the only way to kill him is find his heart. Heart in Azshara's hands. We fight for it in Naga themed raid.
5 - Fight with Davy Jones. We win but "Flying Dutchman" needs new captain.
6 - Who will take cursed place? I have three possibilities.
Daelin Proudmoore - Best admiral in WoW universe. He also linked with Jaina and he was mentioned in Legion DK campaign. In order to get Daelin heroes ventures into Shadowlands OR ask Bolvar for help.
Sylvanas Windrunner - I'm serious. Making Sylvanas new captain of "Flying Dutchman" will remove her from Warchief seat and Forsaken story without killing her. Also it will be ironic - she wants immortality (here you have it) and will be forced to transport dead souls into afterlife (not raise them like she do since Cataclysm).
Taylor - read above.
7 - We go at other side of deep seas and fight N'Zoth.
PS As for expansion title i would call it -
Captain of Cursed Fleet OR
Cursed Fleet of Kul Tiras.