The studio wanted Robert Redford as Michael Corleone. Coppola wanted John Cassavetes, who the studio thought to be too old. Coppola and the studio both wanted the Brit Lawrence Olivier as Don Vito (who would have been brilliant), but he turned them down and then we got the (non-Italian) Brando as a result.
Cassavetes was too old. I can’t imagine having him play Joe College when he was around 43(?). Olivier would’ve been a curiosity, but he didn’t have the beefy menace of Brando. Of course, I’d have loved to have seen Olivier play Brando’s role in “Last Tango In Paris” (especially the butter scene). ;-D