Beasts are mostly included in MO (2 SK, 2 Neutral, 8 MO) and the number of synergies is pretty low. And Soldiers are already spawned by the Ornamental Sword.
What else is still there to be spawned? Alchemy and Items are spawned by Ciaran. Organic by Whispering Hillock, Cursed by Adda.
From his image he is standing in a shipwreck and looking quite mad. So cursed as well as machines would be obivous choices.
The Witcher Wiki views the picture of the leader on Eist Tuirseach pages. Also he was supposed to feature as a leader already in Witcher 3 Gwent. Besides him being the obvious choice I think he really is the leader.
Being the king of Cintra, spawning a Cintra unit might be fitting, but even including all golds being assosiated to it, that might be only Pavetta, Ermion, all Ciris, Rainfarn and maybe a certain other card. So it doesn't seem likely.
Another thing that might indeed work, is that his dead body was retrieved by Skelliges from the battlefield. So he might spawn a card from either graveyard. That would definetly be fitting for the faction.