I'm still pretty intrigued by the idea of a fifth gate themed around music, simply because it fits the line up of concepts for WDW so well(animals, innovation, movies) and because it could open up to a very creative way of storytelling and overall theming.
Hmm some other themes I could think of:
Heroes vs. Villains: think IoA but Disney and centered around worlds of various heroes so bring in Olympus for Hercules, Agrabah for Aladdin, something Marvel related, something centered around The Incredibles etc. + a land that people have been waiting for for so long, a villains themed land. I'd imagine a very cool nighttime spectacular, akin to Tokyo DisneySea's Fantasmic but telling the story around an epic battle between good and bad.
Gardens:
Sooo I guess most of the parks really work on their scenery and especially on the greenery, but imagine a park that is more centered around different gardens around the world. You could have a land themed to European Court gardens with rides based on Disney princesses, a land based on Moorish gardens maybe linked to Aladdin or other stories from the Mediterannean/Middle East/North Africa and Iberian peninsula, something related to alice in wonderland(maybe even tim burton's version!), an Asian garden with e.g. a Mulan ride etc.
Fantastic Worlds/Epic Universe:
Okay so I guess this would be sort of a copy of the concept that Universal is using for their third gate in Orlando, but with the variety of properties that Disney now holds the rights to I think it'd be pretty interesting to explore the possibilities of having a park filled with immersive lands, maybe something related to Marvel, a 20th Century Fox property even(?), a Tim Burton property, Zootopia, Game Central from Wreck it Ralph(with like a sugar rush racers ride, heros duty shooter ride and maybe even a wreck it ralph drop tower or something like that), Monstropolis, endless possibilities here.