Magic Kingdom needs added capacity. Badly. This would best be served with 3-4 more C/D tickets in my opinion. These don't need to cost a fortune... Some relatively simple dark rides with relatively simple facades would do wonders. Magic Kingdom's capacity has not increased in 30 years despite attendance dramatically increasing in the period - especially in the last 10 years. Tron is the first addition in capacity in 30 years (though, Splash is now out for 2 years and even still, I'm not sure what the total capacity of the park is compared to 30 years ago - I doubt the needle has moved much). It also has a 48inch height requirement.
The other parks, especially Hollywood Studios and Animal Kingdom need more things to do. Hollywood studios needs at least 1-2 more lands, each with three rides, one being an E ticket and the other two being large capacity rides. Animal Kingdom would probably be ok with an addition to Avatarland and one new land - the Moana boat ride + 2 C/D tickets would probably do. EPCOT would greatly benefit from 2 D tickets being added to world showcase. Again, these don't need to cost a fortune, but Disney can't build a carousel for less than $100M.