As of Summerset Isle, the problem is is that it seems limited. The size needn't be scaled to the map, but simply being an island would make it feel less free. Nevertheless, from the books I've read in TES:4 it seems like a great, interesting place. It would hold lots of importance to the lore. My second choice would be Elsweyr, because of the variation that could be invested into that, as well as it seems like a good place for a crisis to spontaneously take place.