Of those options he's definitely more of a Brando. He was an artist in various media (including some quirky little animation pieces) and was rumored to be interested in directing.
Considering his willingness to create art for and with little money, I can see him making common cause with the skid row B movie shops. He was already friends with Dennis Hopper, it's not much of a stretch to see him getting involved with Roger Corman and at least utilizing the methods and equipment Corman is associated with.
A model that might be a decent predictor is John Cassavetes, working as a Hollywood actor to finance his own underground films. Dean's desire for control and his already formidable attitude would most likely lead him to falling in with the counterculture.
But it's pretty impossible to talk about the success of one actor/director based on performances we can never see and films that were never made.
So the only interesting twist I can glean from this is that he becomes the first major "outed" star sometime in the mid-late 60s. It's a great boost for the gay rights movement, and gains them a slot on The Great Hippie List of Causes.
He hangs out with Warhol and John and Yoko. Takes a turn with Scorsese in the mid-70s. Goes to Europe maybe, Truffaut and Godard can fight over him with Bertolucci Fellini, Herzog and Wenders.
A brief flirtation with the Studio 54 crowd sobers him up a bit after he loses a friend or two. In the 80s, if he's been at all successful as a director, he settles down and directs fairly prolifically. Or maybe disgusted with Reagan, gets into politics, becomes mayor of some town out West, maybe works his way through the ranks. Dies a retired governor, lobbying for open spaces or gay rights or arts funding for schools.
But again, that's just going off of what other people did.