We like the font, arrangement and colors of "Atlantis Rising" by T.A. Barron (shown below) but are definitely open to new ideas.
Instead of the central, round stone, could be three crystal columns, one emerald, one blue, one platinum. (The three mega-crystals that power Atlantis. They're in an underground white marble "cathedral" surrounded by a maze).
Overall colors could be blues, aquas, golds, greens, even purples. The next two books may use the same images but in different colors.
Back silhouettes of 20 yr old twin sisters (not identical) perhaps overlooking a bit of "High City". Aiela is muscular, charismatic, with long blackberry-colored hair that she often wears in tiny braids, sometimes piled up. Ahna is quiet, wise, small and slight with shoulder-length, choppy, light-blonde hair. They could be wearing a sleeveless tunic and leggings, fancy or simple robes, gowns, etc. The clothing styles range wildly there.
High City has Poseidon's Palace at the center which rings "holy mountain" in descending layers with columns and statues on the exterior. Crystal pyramid temples, high rise buildings with arabesque caps and spires, an oval "stepped" temple, a mandorla-shaped temple topped with a rose-quartz dome, lots of multi-level, "babylonian style" buildings. The three circular canals with four straight canals cutting through them that run to the compass points. The land rings are large compared to the width of the canals and packed full of buildings, courtyards, gardens, markets, streets paved with huge stone slabs. There are "aeros" but they don't fly within the city walls. They are small and triangular, or sometimes, the old "bullet-shape" style. North of High City is Old Forest with gigantic trees, east is a short distance to the sea with an outer harbor (and an inner harbor just inside the City walls), west and south are suburbs, orchards, farms, etc. Further west is a central range of mountains. None of this has to be in the image. Or just one or two features could be. The Twins and the Crystals are central to the story.
The mood of Book One is light and romantic, an intro to all the wonders and color of Atlantis, the girls becoming. (Darkness enters in Book Two. Book Three is the finale.)
The style is fantasy, but with historical (grounded) elements.
The other pictures shown below are indicative of depictions we liked.