Its clear now that this dlc is not of one race but multiple, given the various refferences in this teaser. The fact that this letter is addressed to an individual most likely represent one faction but reffers to "rivals" implies that there are numerous other parties, all competing and likely from different races with different motives. Lets examine these potential races.
The most obvious first I'd argue is the paychest behind the letter, and the name of the individual writting it. This points to a human in a structured Navy which can most likely refference DOW.
The shattered minds is clearly a refference to an undead faction, the Vampire Coast, what with undead being mindless thralls to their Vampire/Necromancer Master.
The "Syren of the Storm herself" is an interesting title, as othet have pointed out, no character herself has been given this title in Warhammer Lore. The title itself doesnt seem to fit a Dark Elf Captains title (its not menacing enough) and it definetely isnt a title of a Dwarf Slayer Pirate Captain, so it has to be humanoid.
That leaves us with the most likely of candidates, a Sartosa Pirate Lord.
This leaves the fourth spot however, unoccupied. Some might speculate that anyone of the 3 factions may get another lord, but I suspect something different. I beleive that the answer may lie with the origin of this "magical item" to begin with.
Given that it is a magical artifact of great significance, I'd wager that those who live wherever it may have originally come from would be interested in retrieving it. The only question is establishing where such a powerful artifact might have come from. I beleive the answer is obviously Albion, a land home to many ancient and powerful artifacts created by the Old Ones themselves.
Given aswell the likelhood of us seeing Luthor Harkon, a Vampire Coast Lord driven mad by a curse placed on him by an Old Ones Artifact and his obsession with looting Lizardmen Temples to find an Old One artifact to cure his madness, I beleive that this only reaffirms the likelehood of seeing an Albion army represented in the campaign, if only to compete with Harkon.
The liklehood of Albion becoming a seafaring race is slim, but seeing as the Truthsayers have sailed to the Old World and to Ulthuan across the sea of Chaos to seek Allies to defeat Belagor then I imagine its plausible that they can build ships to some extent.
Whats more likely is that many campaign missions may take the other lords to Albion in search of artifacts and treasure, and the nativs will have to repell these invaders.
Given the amount of time CA has dedicated to this dlc and the amount of time the Art department has had to devote to it in perticular as CA themselves stated, I beleive we will be seeing many different factions represented in this dlc, and whilst they may not have huge rosters, I beleive they will have enough to be interesting.