As much as I like tropical islands, I think it would be a mistake for DVC to invest on one. Just look at the trouble they're having selling Vero.
There are already plenty of choices from other resorts and that would be a lot of competition. I prefer Hawaii, others prefer the Caribbean.
Castaway Cay is very nice and the island is bigger than it looks (they only built on a small portion of it), but it would be prohibitive to build a full fledge resort there. There's no good source of water. No electricity (except generators), no good sewage, etc. It would cost a fortune to build any kind of even halfway premium resort and just manage the utilities, air-conditioning, water, food service etc. Everything would have to come in by boat. Yes they could use the Magic and the Wonder, but it still isn't cheap. (The only exception might be if they did a very primitive resort, maybe on the scale of Tree house villas at WDW, that had more of that Robinson Cruso feel, and kept it to only a dozen units, then they might have something. But then it would be too small for most DVC members to even use.)
While it might have been fun to have a DVC resort on a tropical island as one of our DVC non WDW site choices, , we'll just have to settle for having exchange privileges at many of the other non Disney resorts already available all over the world if we want to use points for a stay.
I voted no, strictly from a practical standpoint. If DVC gets into financial trouble building non-profitable resorts, we'd all pay for it one way or another.