I created a thread here once about movie projects that Paul tried to do and/or roles that he turned down, at
There's info there about the Asimov project and more info about the Sci Fi project Paul wanted to do with Gene Roddenberry. Paul asked Gene Roddenberry to develop his ideas into a script after he rejected Isaac Asimov's story.
As far as the story Isaac Asimov developed for Paul & Wings, it was called "Five and Five and One"
Some college students found an original manuscript of the story and made a radio play of it. It's not that great, but gives you an idea of what the story would have been like. The first part is on YouTube at https://www.youtube.com/watch?v=pM-dwpCs6jU
This is a summary of the story, from http://sciencevibe.com/2015/11/17/the-cool-alien-movie-that-paul-mccartney-and-isaac-asimov-didnt-make/ (Paul, Linda and Wings would have had dual roles as the aliens, and their human counterparts. I remember when I listened to the radio play that there's another character, from earth, that they take with them, who is sort of their guide.)
... McCartney’s big idea was about a band who discovered they were being impersonated by a group of extraterrestrials. Asimov wrote up a treatment called “Five and Five and One.”
Asimov’s story featured parasitic aliens who crashed on Earth and had an urgent need to find suitable hosts to invade. His story had them trying lizards, cattle, before settling on humans were a better choice. Once they heared music for the first time, the Alien-Human transformed to eagles so they could fly all across and discover the right musicians to inhabit.... Turns out that McCartney hated the idea and rejected it. Turns out that Sir Paul was disappointed that Asimov hadn’t used any of the dialogue the famous Beatle had already written. Asimov, for his part, thought that McCartney couldn’t recognize “good stuff” when he saw it.
Question: Is that a movie you would have wanted to see?
As a side note, Paul was also offered the leading roles in "Romeo and Juliet" (1968, as Romeo of course!), and "Brother Sun and Sister Moon". The Beatles also were developing a project where they would play the 4 main characters in "Lord of the Rings" but J. R. R. Tolkien didn't like the idea, and wouldn't give his blessing to it. Paul would have played Frodo Baggins, Ringo would have played Sam Gamgee, John would have played Gollum, and George would have been Gandalf.