The Other Darrin: Jackson Bostwick (Captain Marvel) was replaced at the start of the third season by John Davey.
Reality Subtext: Whenever Superman and Captain Marvel are in the same comic together, it's a pretty good bet one of them is going to punch the other. This is a reference to their famous court battle, where DC sued Fawcett over similarities to Superman.
What Could Have Been: According to Alex Ross's Rough Justice art book, he proposed a redesign of the Marvel Family which would've slightly aged Billy to that of a teenager, and modeled him and Captain Marvel after their counterparts from the TV show. Ross's revision would've also included a reworking of Vulcan, the Expy of Black Lightning from Superfriends, with a backstory connected to Shazam. This direction was pitched when DC's decision to make Freddie Freeman inheritor of Shazam's power had been decided on.