Both the Tycoon and Isola frameworks would lend itself to lots more variations. I am fascinated by how different Plant and Fish are to play, even though I thought they would be similar- raise variations and sell them. But the breeding mechanisms turn out to make the games very different. Have you thought about Bird Tycoon or Insect Tycoon, again getting supplies from Isola so you could keep the magic motif?
And on Isola, who is finding and shipping all those fish and plant seeds? In Village Sim2, maybe we could send our Master Scientists on expeditions into the interior and/or under water to find old and new types? It would take some skill to outfit an expedition with the right mix of hunter/gatherers, scientists, builders, and healers to keep the group from perishing along the way and make it back with our new catches. Not to mention avoiding taking along the eager-breeder villagers! Lots of potential for Island Events, too.