The way the landmasses are set up, I can't image that any one of them would be larger than the sum of the British isles. So, building off of Vadin's idea, why not make this a water-world. A massive, global flood forced people to the sea. Some lived in flotilla colonies; some lived in domed cities beneath the waves; some, through magic or natural adaptation changed to live beneath the waves.

Now, though, a thousand years later, the waters are finally starting to recede. These islands would be the tops of the tallest mountains of the old world, and the first to crest the waves again. Some of the various people of the ocean have taken notice of this, the water-breathers donning tanks and magical amulets to survive the air, the flotillas dropping anchors and building docks for the first time in a millennium, and the mages of the bubble cities walking ashore, plans in mind to reclaim their ancient heritage.

What do you think?