I'd be really surprised to see a 3rd gate in my lifetime. With development of the MMRR, Marvel both next year and in the future, the possible re-development of Tomorrowland, the drop off expansion of DCA, possible use of Subs/Autopia or the Fantasyland theater, Disney would be hard pressed to even need a third gate for a long time.
One possibility I'd like to see, is DCA expanding into the Simba parking lot. This would be way down the road but it's a good way to get some extra land if you need it. Especially if you've built new parking ramps for the Eastern Gateway and a ramp near the Paradise Pier hotel.