That is a tremendously detailed and passionate vision for a potential Back to the Future Part IV and the return of The Ride!

You've created a comprehensive proposal that covers the storyline, character arcs, a full cast and crew list, soundtrack additions, and a plan for how to re-theme the current Simpsons Ride back into a revitalized Back to the Future: The Ride.

Since your proposal is an extensive creative work, I will summarize the key elements and address the feasibility of your ideas based on the existing realities of the franchise and theme parks.

🎬 Back to the Future Part IV: Proposal Summary
 

Your proposed story, featuring Marlene McFly (Dove Cameron) as the new lead, is a clever way to continue the saga while incorporating elements from the original trilogy and the continuity of the theme park ride.

Key Story Elements:
 

New Protagonist: Marlene McFly, Marty's daughter, takes on the mission to save Doc Brown and finish her father's journey.
Dual Timeline/Plot: The story blends a sequel plot with a prequel plot:

Sequel: Marlene travels back in time to 1905 to save Doc Brown (Christopher Lloyd) from Buford "Mad Dog" Tannen.
Prequel: This plot ties into the Back to the Future: The Ride lore, showing how 1955 Biff Tannen, informed by 2015 Biff, travels to 1991 to trespass at the Institute of Future Technology and steal the DeLorean, directly leading into the ride's original pre-show.
Special Appearances: You've planned a final, touching reunion scene for Marty McFly (Michael J. Fox), his family, and Doc Brown, with the added context of bodyguards to respectfully incorporate Michael J. Fox's health condition.
Tom Holland as Marty McFly Jr.: You include a sub-plot that re-imagines the hoverboard chase scene from Part II with Marty Jr. and Griff's gang, featuring Chloe Lang.
 

Feasibility Check:
 

Official Stance: Robert Zemeckis and Bob Gale (the original writer/director and writer/producer) have historically been very firm that there will never be a Back to the Future Part IV as long as they hold the rights. They feel the trilogy is complete and are opposed to creating a sequel purely for financial reasons. This is the main obstacle to any official sequel.
Cast: Your proposed cast list is ambitious, bringing back nearly every living major cast member, which would be an incredible feat for fans.
The "To Be Continued..." Edit: The idea of editing the end of Part III from "The End" to "To Be Continued..." is a creative nod, though it's highly unlikely that Universal would alter the original film master to accommodate a new movie, given the directors' fierce protection of the original trilogy.
 
 

🎢 Theme Park Ride: Proposal Summary
 

You are advocating for the permanent closure of The Simpsons Ride at Universal Studios Hollywood and Florida and the restoration and modernization of Back to the Future: The Ride.

Key Attraction Changes Proposed:
 

Restoration: Completely restore the attraction to the Future Technology Institute theme, including the DeLorean vehicle and original queue and preshow.
Removal of Simpsons Elements: Permanently remove all elements of The Simpsons Ride, including the water effects, baby powder, buzzing seats, and character meet-and-greets.
Modernization: Update the ride with a new film footage clip featuring Marlene McFly (Dove Cameron)to tie the ride into the new film's lore.
Expansion: You suggest expanding the "Springfield" areas into a "Hill Valley Expansion" and bringing the ride back to Universal Studios Japan and, for the first time, building it at Universal Studios Singapore.
 

Theme Park Reality:
 

Intellectual Property (IP) Status: You are correct that The Simpsons is now owned by Disney (via their acquisition of 20th Century Fox). However, Universal Studios has a long-term licensing agreement to use The Simpsons in its theme parks. This contract is what keeps the ride and the surrounding Springfield land in operation.
Popularity: Despite being older, The Simpsons Ride and Springfield area remain very popular and financially successful attractions for Universal Parks, making a permanent closure unlikely in the near future unless the contract is not renewed or a major, successful new IP becomes available.
Fan Demand: There is a constant, strong demand from classic Universal fans to bring Back to the Future: The Ride back, but the operational costs of the motion simulator and the need to refresh the outdated film make this a major undertaking.
In summary, your proposal is a comprehensive and exciting vision for how the Back to the Futurefranchise could be revived and celebrated by fans across cinema and theme parks, though it faces significant hurdles from the original creators' stance on sequels and the current business contracts governing the theme parks.