Anything's possible... but this rumor really sounds like wishful thinking.
First, consider the cost and complexity of building a major structure where all building materials must be loaded and unloaded from trucks to barges and back to trucks -- or where trucks need to be ferried over. And you can't just drive the crane onto the construction site either.
Next, once completed, consider the logistics of bringing food (including refrigerated and frozen items) and other supplies to such an island resort. Trash removal becomes much more difficult and costly. Even luggage handling becomes complex and costly compared to a resort where you can drive up.
Then, consider fire safety and emergency services. The island would have to have its own fire station, as well as emergency boat service with transfer to ambulances.
Next, consider that the guest check-in and dock would need to be fairly distant from Discovery Island, such as by the Wilderness Lodge -- or that they would have to cut a road to some place near the Pioneer Hall / Crockett's Tavern area of Ft. Wilderness. For guests, the boat ride to the island would be fun at first, but it would get old fast.
Sure, if Disney could charge a high enough rate, they could pay for the additional cost and complexity of building and running a resort on Discovery Island. But now-a-days, Disney doesn't seem interested in projects that require premium construction costs and premium operating costs.
For guests, a Discovery Island Resort could be a nice experience, at least while at the resort. And there could be direct boat service to the Magic Kingdom. But for other destinations, guests would first need to wait for a boat to the Wilderness Lodge or the Contemporary or possibly to the check-in builidng -- and then wait for a bus to their destination. Not very convenient, is it?
There are still plenty of other undeveloped waterfront locations on WDW. If Disney decides to develop another deluxe resort -- or even an ultra-deluxe resort -- there are plenty of other good places to build.
Anyway, this is a fun rumor, but I consider the chance of it actually becoming a reality to be near zero.
