Gotta say, Disney in the late 1990s and early 2000s was significantly better.
Bring back:
Fastpass or Fastpass+
Magical Express
Resort-specific mugs (If you really wanna get picky, restaurants had site-specific napkins, too. None of this generic brown stuff)
Tarzan Rocks
Epcot as an edutainment park. Wonders of Life, Food Rocks, Kitchen Cabaret... just some of the memories. Upscale the uniqueness and educational value of the countries in WorldShowcase
Afternoon parades in ALL four parks (or evening, at Epcot)
Osborne Lights
Adventurers Club (Kungaloosh!! Maybe have some exclusively-adult clubs. Disney is not only for families and kids!)
Improve:
Food variety and quality.
Customer service. Seriously, Disney. The costs could be justified due to the outstanding customer service and uniqueness of your parks. Everyone really felt like a welcomed guest. Not anymore. We are all now just beans in the counters bags.