Wow. You've given us practically the world of literature to choose from. Hmmm. Some things that come to mind (okay, I can't pick just one) are:
1. I'd like to be a bystander on any of HST's adventures.
2. I'd like to be able to write a timeless classic like Peter Pan.
3. I'd love to be able to churn out novels like Stephen King, none truly great, but all of them entertaining and eagerly awaited by my audience.
4. I'd like to be friend/lover to JM Barrie, the one who would have eased his pain (and who would no doubt have destroyed his creative angst in the process
5. (Some days) I'd like to be Candy (Terry Southern) or Chenault (Rum Diary) - the physical object of men's desires.
6. (Some days) I'd like to Jack/Sal or Neal/Dean (or even Peter Pan), self-absorbed and looking for the next high.
7. (Some days) I'd love to be a kid (Charlie) in Willie's chocolate factory.
I could go on but you get the picture. Alas, "I yam who I yam" as the great philosopher Popeye says.