Read up on a wiki about Issylra and Maequet and there really don't seem to be much on either one of them honestly, compared to say Wildemount or Tal'Dorei. A couple of city states and that seems to be it. Neither seems to be meaty enough for a Campaign Setting Book.
Agree with
@DEFCON 1 , Issylra, Maequet and the Shattered Islands have just as much written about them in the Tal'Dorei book as Wildemount does. I see no reason why Mercer could not flesh them out more for a future campaign.
The Shattered Islands is described to have 43 islands; that's honestly more than enough material for a campaign.
The Shattered Teeth
Should someone wish to sail the dangerous waters southeast of Tal’Dorei, past the ever-present fog bank called the Fool’s Curtain, one might discover themselves adrift among a cluster of forty-three islands referred to as the Shattered Teeth.
Varying in size from small, mile-across reef-toppers to the large, city-holding islands like Ruukva, this cluster of broken lands contains two combative societies: the Ossended Host, an enormous fisherman culture of isolationism and self-empowerment through the worship of the power of dreams and nightmares, and the Wanderman Assembly, a centuries-old trade company that, upon being stranded across the Shattered Teeth after a hurricane, began a somewhat brutal capitalistic society that poorly masquerades under the banner of honor and brotherhood. This tension has led to bloodshed in recent years, and each group is paranoid the other is bent on the destruction of their way of life. What brims beneath the surface of this conflict is the nature and source of the dreams the Ossended Host have been harnessing for generations, and what that answer may mean for the struggles of the people of the Shattered Teeth.
Inspirations: From a lifestyle and architecture standpoint, the Ossended Host bears a 10th century Japanese fishing village flavor to most townships, with the Wanderman Assembly taking inspiration from 16th century British colonies in the Caribbean.