All islands will be beatable eventually. I'm not one of the devs, but I'm happy to help with explaining things. I don't have any way of knowing which islands are a priority. I thought it was going to be in release order, since Early Pop and Shark Tooth were some of the first ones, but then there was Zomberry, so it's hard to tell. My estimate is that they'll all be finished in a couple of months. In hindsight, maybe he should've given this project a little more time to cook, or at least have called this the beta.