South seas and Azsharra.

Zones: Nazjatar, Ruins of Undermine, Ruins of Kul Tiras, Zandalar, Plunder Isle, Broken Isle

Horde and Alliance big town : They will not have big town but a naval, air and sub fleet, which will moved with phasing, to an isle from another.
Alliance will try to make deal with the south seas pirates, while Horde will negociate with Zandalar Troll, each faction trying to improve relationship with two faction which are not likely to accept them.

I'm not telling that South Seas Pirates will be part of the Alliance, and Zandalar will be part of the Horde, they will just be neutral faction for one, and hostile for the other.

New races :

- Naga for the Horde, Azsharra and her people or a separate faction of rebel Naga trying to free themselves from the Old God by making alliance with the Blood elves and Forsaken. A naval race for the Horde. They could have a little home quarter, the hidden city of Atlantiss, on the large of Silvermoon.

- Vrykul for the Alliance, a small island could be discovered by the explorer league, with an ancient Vrykul tribe living on it, exiled from Northrend for their refuse to sacrifice their anormal children.. They will be the ancestor of humans, and could use the Asgard and Mitgard mythology.
It will be a naval race, hostile to Naga and Kvaldir, with link to the Dwarf and Human.

New class if no new races : Bard
Mix between mage and rogue for fighting style, with more charm, illusion magical knowledge, and less stealth style, and more combat ability like feint, jump, acrobat person. They could have a connexion with South Seas Pirates for the Alliance, and Zandalar Trolls for Horde, having a lot of stories to tell !