It's sort of disappointing to see The Great Movie Ride go...since I rode it several times when I was younger. In my opinion, Hollywood Studios is the most "nolstalgic" out of the parks besides the Magic Kingdom. Although so much as changed, they have kept several experiences that were there, such as Muppet-Vision 3D, The Voyage of the Little Mermaid, Beauty and the Beast, and of course, Fantasmic!
A Mickey Mouse ride sounds cool! If they plan on keeping the building, I have a feeling that this ride might be a Spaceship Earth-esque ride about Mickey Mouse and his friends telling the story of the history of Walt Disney and his legacy and possibly acting it out.
I know what you're thinking: "Hollywood Studios already has an exhibit on Walt Disney that is basically what you described, except with the 'Mickey Mouse and his friends acting out the history of Disney' part!" I am well aware of that, but over the past few months/years, there have been several clues that One Man's Dream may close soon, with the dead giveaway being the fact that the theater that used to show the movie about the history of Walt Disney is now showing previews for current Disney movies, such as the live-action remakes of The Jungle Book (when I visited, but I didn't actually see it since I thought it would be a waste of time) and Pete's Dragon. My theory is that One Man's Dream will close, and (this isn't likely) a majority of things in the exhibit may be moved to a post-attraction area or an area near the Mickey Mouse ride.