like you, I'm loath to introduce in that city other sources of power (and corruption), as a dark gem or such. My reason is that there was already the terrible Marsh Bell, and for me as LM it was the focus of the darkness in that place. One of them paid the highest price to destroy it, if I create another source of power I'm diminishing their effort and somehow cheating them of their victory.
But a Queen? Yes, I think there is definitely a place for a Queen in the sunken city

BTW: I skimmed through The Heart of the Wild... there is no additional material on the Marsh Dwellers and their city, isn't it?