The fact that someone has "already done it" doesn't mean much in terms of biopics. I mean, I look back at Biopics of the last couple of decades, and several of them had an "eh" version done beforehand. "All The Kings Men" (a story of Huey Long) will be the fourth movie that I know of about Huey Long (Kingfish came out years ago with John Goodman, and there was a TV version). Etc.
More then that, with so many cable networks, there will be tons of biopics that will be redone because cable networks will have to do them for the programming So I'm just talking about the ones that I think would make an effective transition to the screen 20 years from now, or after the people they are based upon are dead.
In my grogginess this morning I heard a name on the radio that is a no-brainer upon reflection: Robert Downey, Jr. I've heard his childhood was rather interesting as well. I think another person who would qualify is Johnny Depp - both actors I've been aware of since I was a child and who fit into my "generational window."
Since music biopics are so popular, I see Bono and Bob Geldof in the future. I'm surprised when Jerry Lee Lewis and Buddy Holly had biopics that Chuck Berry hasn't had one. And Brian Wilson should have 1 for each personality.
On other entertainment fronts, a serious pic about Arnold Schwarznegger would be worthwhile in a few more years. Woody Allen's life would make a terrific black comedy. And, I'd like to see Milos Foreman take a shot at David Letterman's life.