I am not that familiar with map extensions.
My plan is to use the Island, add many caves, change existing, change the vulcano, add another with crater, add landmass around the island, change the snow biome, and change the existing island landscape somewhat.
Also change the weather system. (more extreme)
It was my impression that such changes would go beyond a ‘simple’ map extension and would require a redevelopment of the Island map as a whole.