When our family went to Disneyworld back in '03 we were talking to a Disney Cast Member inside Disney's Yacht Club Resort and she said that Disney was planning to create another hotel, a "Sea World Themed" Aquatic Park, and a "Roller Coaster Thrill Park" to compete with Universal Studios. She mentioned that it would come in the next decade. Not sure if this is true or not. Sounds kind of accurate since they did open The Seas with Nemo and friends, and Disney's California Adventure (Already opened in '03). Even if they did open a 5th, 6th Gate it would be under wraps for quite some time.
They can only keep it a secret for so long before someone knows SOMETHING is going on, especially for something as big as another gate. Take Bay Lake Tower - you know its there, I know its there, all the monorail drivers know its there - but officially there is nothing there yet.
It would take at least a couple years I would think to build another full-sized gate anyways.
But all three of the mentioned things have been rumors (all mentioned by Jim Hill at some point I'm sure) that haven't had a ton of meat to them so far.
It's no secret that Disney has been considering ways to compete with Discovery Cove, but it would be unlikely that they'd just copy it. And if they really wanted to, they could just buy it now. Mostly they seem to be looking at an more land-based, adventure-style upscale attraction instead.
A true 5th gate has been rumored for a while to be a thrill-based park, typically revolving around a Villains theme, but it seems to alternately be on and off the table, and a pure thrill park doesn't match Disney's style.
And I'm sure Disney is ALWAYS thinking of another hotel - but they have to have a reasonable expectation to FILL that hotel before they commit to building it, which would probably prevent a green light on it given the economic climate. Plus, they could do with finishing the POP first.
DVC at least is building, at (assumedly) Bay Lake Towers, Grand Californian, and Hawaii. DVC's business model practically REQUIRES them to keep building until near 2042 to keep the revenue stream.
And there is the Four Seasons hotel, but Disney isn't actually building that...