Your best guess now would be that Brad Bird and Damon Lindelof's top secret Disney film is about Tomorrowland, the theme park "world" because that's just what it's called. The title was announced by Disney just now. They use a picture of the "world" on their D23 website in reporting the news themselves, so I think it's a fair connection to make.
The film was teased last week an image of various apparently random items inside a box. Amongst bric-a-brac were pictures of Walt Disney. Does this suggest that George Clooney might be playing Disney in a film about the creation of his first park? That could mean it would actually be set in 1952, when planning for the park was underway.
The only two other roles we know anything about are from a casting call we received. They're looking for a boy of around ten or eleven and a "beautiful" teenage girl. Visitors to the park, maybe?
I'd guess so. And I'd guess they, and Walt Disney, get caught up in some sci-fi adventures.
And most likely they'll be shiny, polished sci-fi adventures too as Claudio Miranda, the cinematographer of Life of Pi, Benjamin Button and Tron: Legacy wil be shooting the film.
Footnote: This official title means that Disney are currently developing Jon Favreau's Magic Kingdom and Brad Bird's Tomorrowland. Should we expect that films called Adventureland, Fantasyland or even Epcot are in the pipeline somewhere?