pastajoe wrote:So if Cap is a future version of Billy, what will happen when Billy is an adult? Will he always be Cap, or if he says Shazam! will he turn into the old Wizard, or what?
It's not that he's a future version, it's that he's what Billy will look like when he's an adult, that's never really been in dispute, why it keeps being brought up however doesn't have anything to do with what we're talking about.
Captain Marvel/Captain Thunder was really Parker's creation, from the same interview:
I didn’t create any of Fawcett’s characters—I was just the first person to put them into visual form. They were conceived in Parker’s mind; I was just the doctor who held them up and slapped them on their bottoms to make them draw their first breaths.
Heintjes: Did you ever make suggestions about stories you were given?
Beck: Rarely—I wasn’t an editor. I once suggested that instead of killing Mr. Mind off, we could have him turn into a beautiful butterfly and fly away, but that idea was killed by the editorial department.
Heintjes: I guess justice wouldn’t have been served. But despite the real challenges in illustrating a story, you obviously believe in the primacy of the story.
Parker wrote all the issues up to 1942, then was drafted. Beck didn't take full control until after National started to distribute the character in 1972.
Golden Age Cap under Parker was definitely much different than what Beck did in the 70's and the post Silver Age incarnation of the character.