Well, sure--when an actor gives an iconic performance in a great movie, as did DeNiro in "Goodfellas," it's really hard to envision anyone else in that role. Could anyone envision someone other than Charlton Heston as Moses in "The Ten Commandments"? Still, I think Pacino had the acting chops to bring off Jimmy if he'd taken the role instead of DeNiro.

Here's a story from "The Godfather":

I bought the three-VHS, "25th Anniversary" set of the GF Trilogy when it was released. Each movie began with commentary from one of the principals. "The Godfather" tape had Mario Puzo in a deadpan narrative. He said that, when Paramount announced it would film his novel, he heard that Danny Thomas (!) planned to buy Paramount so he could play Vito Corleone (!!!). "I was a little disturbed by that," Puzo deadpanned. He said that his agent had Brando's mailing address, so he sent Brando a script and asked if he'd be interested in playing Vito. Brando wrote back that he'd be interested but needed to see who'd be selected as director.

I never saw or heard that story anywhere else.