We've heard it before. Actors who were considered for a certain role didn't play it, but the ones who got it instead feel completely natural. Popular examples include Will Smith, who was considered for the role of Neo in The Matrix films, which Keanu Reeves made iconic instead. And it's hard to imagine anyone other than Ian McKellen as Gandalf, even though Sean Connery turned down the role.
The cast of the Guardians of the Galaxy films could have been very different too, and James Gunn has said the following in Marvel Studios: Assembled documentary on Disney+ about the film: "When I first went to Marvel and I pitched 'Guardians of the Galaxy,' I brought a little book. I had worked very hard trying to put this little book together of all the little pictures and photo references, and all that stuff. And in that book, there were all the characters listed and who I thought should play those characters. And of course, none of those actors who were in that book ended up playing the characters, except for Zoe Saldaña, who I met and loved from the beginning."
It is not revealed which names Gunn had in mind instead, but the casting of Chris Pratt as Peter Quill, Dave Bautista as Drax, Vin Diesel as Groot, Bradley Cooper as Rocket and Karen Gillan as Nebula all seem like very obvious choices for the roles. Although it was only Zoe Saldana as Gamora who was on Gunn's list and who also ended up in the films.