Here's the thing. Any additions to the other three parks are still going to be bad for Magic Kingdom who is struggling the most with capacity. While we have made fun of getting excited over new restrooms and the Hub project, those are the things that they needed to do to ensure that when they do add Avatarland and other additions that MK will be able to take it. There in also lies the problem, why does MK attract so many and the other parks don't. The other 3 parks need to get on par with Magic Kingdom. It's obviously a tug of war.
One thing I think it will ultimately come down to is building a resort in South America and improving Disneyland Paris Resort. WDW sees so many visitors from South America. While it is great for our economy, building a resort closer to that market may help with crowds here at WDW. Even improving Disneyland Paris and especially the Walt Disney Studios Park may just encourage more Europeans that what they have is great and good enough to visit. WDW does get guests from Tokyo, but not as much because they already have a great set of parks.
When it does come to rides, we need more PeopleMovers and PeopleEaters things that will get them in and get them out all with continuous loading, minimal stops, but yet still a great attraction and story. Horizons was a perfect example. It has an amazing capacity. Other things do make a difference such as the extremely long load & unload area at Harry Potter. The extra long length ensures that there is more time to board resulting in less stops to keep things moving for everyone. It doesn't matter what "ticket" these new attractions are, so long as they have capacity in mind.
They also need to get rid of or update under-performers. The parks have some areas that really could use something new to help eat up some crowds. Stitch's Great Escape, Carousel of Progress, Tom Sawyer's Island, Hall of Presidents, Country Bear Jamboree, Tiki Room, Swiss Family Treehouse, Ellen's Energy Adventure, Circle of Life, Captain EO, Journey into Imagination, Impression de France, Reflections of China, American Idol Experience, Sounds Dangerous, Magic of Disney Animation, & the Backlot Tour can all hold more people, but don't attraction lots of crowds for various reasons. Not all of these need to be changed, but some could benefit from it.
Less clutter is another thing that can help solve capacity issues. Look in the parks and you'll see lots of excess merchandise and food carts. Yes, these are all money makers, but they are also blocking pathways, causing bottlenecks, making crowded conditions more chaotic. There are plenty of untapped dining locations to help with snack stands and there are also plenty of gift shops.
More organization is another big thing. Storybook Circus has this thinking in mind. Wide paths, stroller parking that is tucked away but designed into the plan, lots of places to sit, added ride capacity, huge gift shop location, and great theming. It will still feel crowded on a busy day, but they kept crowds in mind.
Continue to make upgrades to infrastructure by expanding walkways, improving restroom/restaurant/merchandise facilities, and finding ways to organize guest traffic flow without a) it gets to the point where all guests feel like herded cattle and b) that that park pathways turn into litteral roads complete with stop lights, traffic signs, and designated directional traffic lanes.
