Studio interference could be perceived as a positive or negative thing, depending on how an individual sees it. At times, it aids in enhancing the quality and making the movie reach its desired milestones. It can also sometimes mess with a genuine creative vision and make the final product a critical bomb, potentially hurting its box office performance and revenue. Here are two examples of Marvel movies whose plots and endings were rewritten and changed—for the good or bad?—it's up for your interpretation.
Fantastic Four (2015)
Fantastic Four (2015) was supposed to serve as a reboot to the franchise but failed miserably, soon claiming the title of the worst superhero movie to be made. Overall, the movie was highly criticized for its abrupt ending and awful origin story with no character development, gloomy tone, and lack of imagination. Since in development, the movie faced ample problems as initially Chronicle filmmaker Josh Trank and screenwriter Jeremy Slater's opinions clashed on the tone of the movie; the latter wanted a more comic-book-inspired tone, including Galactus and Doctor Doom, which wasn't agreed upon by the former as he had his ideas. This led to Slatter eventually exiting the project.
The first cut received a different ending as Trank's script was rewritten while filming, but the efforts went in vain when 20th Century Fox was not satisfied with the final output and demanded changes and reshoots. As per sources, there were creative disagreements between studios and Trank too; this led to their two separate versions being worked on. The studio also had to piece together parts from the original script as well as conduct new rewrites to form an ending. As the studio disregarded Trank's cut and suggestions, it made modifications on its own as required and released the movie, which ultimately was deemed a critical bomb.
Thor: The Dark World
Thor: The Dark World had been critically panned not just by audiences and critics but also by director Alan Taylor, who believed the movie lacked creative control. During the editing and reshoot stages, the movie was altered a lot. (via THR) The movie became the lowest-rated one on Rotten Tomatoes until Eternals and Ant-Man and The Wasp: Quantumania happened, which stole its spot. During the final scenes, Thor goes to Asgard and decides in front of Odin (who is Loki in disguise) to give up the throne and later reunite with Jane Foster on Earth.
In an alternate ending, as Thor wakes up in Jane Foster's apartment, he realizes his father Odin is beside him and apologizes and acknowledges his worthiness for the throne. Later, a conversation about their relationship's future takes place between Thor and Jane, which concludes with a breakup, and the final scenes see Thor taking up the throne. In this particular ending, Loki's story came to an end. But why did this perfect ending change? Well, when test screenings were conducted, the audience found it hard to believe that Loki died. So, to continue the God of Mischief character story in the MCU, the studio decided to go with an altered ending while also maintaining Loki's fake death trend.