MK is in kind of a weird position right now. Any attraction you add that will increase capacity will in turn attract more people and defeat the purpose.
It is something you see in road design called induced demand. When a highway gets clogged up it is logical to add lanes to increase the capacity, but what happens is more people take the new and improved highway and the same problem starts all over again.
One of the ways you solve the problem is to give people other options.
Disney seems to be doing that very thing right now. AK has a big expansion and new night time show just around the corner, the Disney Springs Expansion is nearly finished and DHS is getting what could be one of the most popular theme park lands in history. Hopefully major work at Epcot is somewhere on the horizon as well.
This should, when complete, help to move guests from MK to other parts of the property and help relieve some of the strain on MK.