It's an interesting question. Because a MCU film starring Taylor Swift would start breaking some box office records. If it's structured like The Bodyguard ( how the script essentially shielded Whitney Houston from extending beyond her practical acting range) , then IMHO it could work. I suppose it would be tricky to take that angle since so much of it has been covered by Starlight/The Boys, but it's not inconceivable that a story about a "super hero" who lost their powers but is still world wide famous could really grab an audience. Obviously it's not the standard assembly line MCU film, but you have to play to the strengths of your performers and their limitations.
A more dramatic take on how isolating it can become to have no privacy and to be pressured to meet the expectations of the public, that can be pretty compelling if the story is simple and the audience can relate in some fashion to the lead. No one knows what it's like to be Taylor Swift, but everyone knows what it's like, at some point as an adult, to be utterly alone and feeling completely untethered.
This is where, IMHO, Joss Whedon doesn't get a lot of credit ( and even less so since his scandals, etc, etc) He puts his performers in the best position to succeed. He lets them play themselves but highlights the most useful parts to help the narrative. He focuses on playing to someone's strengths. But he's a strong enough writer to do that, most people in Hollywood can't write their characters as well as Whedon.
Whedon could have saved Captain Marvel. One could argue it would have been a better fit for him than the Avengers films.
From a marketing standpoint, a major film might be too much exposure for Swift though. It would make boatloads of cash, but it might create fractures in her overall brand. She fits the algorithms, but she doesn't naturally elicit empathy from the mainstream. And she's likely not a good enough actress to bridge that gap. Just some thoughts.
