The FAST & FURIOUS Universe would have been a lot different had The Rock never walked through its doors. And, heading into FAST FIVE, he wasn't supposed to.
The character of Hobbs was originally drawn up for someone else, as revealed by Vin Diesel during one of his recent Facebook videos. It was meant for Tommy Lee Jones.
"Back in the fifth film, I was asking ‘Who should I work with?’ The reason why we brought Dwayne Johnson into Fast Five was because of you. There was a girl named Jen (maybe, Jan?) Kelly who said, ‘I would love to see you guys work together on-screen.’ And so, we gave a role that was initially written for Tommy Lee Jones but we gave it to Dwayne, and he shined in it."
It's hard to imagine Tommy Lee Jones exchanging blows with Diesel at any point in the series, but it's pretty apparent the character of Hobbs was constructed to be something more in line with Jones' Deputy U.S. Marshal Samuel Gerard from THE FUGITIVE. It certainly would have brought a different dynamic to the films for Dom and his crew to be pursued by such an intelligent foil, but I think we can all agree that The Rock's participation has worked out thus far for the franchise.