Fallout co-creator Tim Cain has designed a secret sequel that no one will ever see: "I'm never ever going to talk about it"

gamesradar.com:

Tim Cain, one of the lead developers on the first Fallout and the man behind its original design, says he has a secret plan for a sequel that nobody's ever going to see.

"Just to make people salivate - I've told people this because I'm never ever going to talk about it - I already designed a Fallout," Cain said on a podcast with YouTuber TKs-Mantis. "Nobody's ever seen the design - I didn't even tell Leonard [Boyarsky, a key dev on the original Fallout] the design - because if it ever gets made I will know it's a coincidence. But I'd already thought of what I wanted to make as my next Fallout. But it's never been made. I don't even see things going that way, so it just sits in my head." 

If you're not familiar with the history of Fallout, it's a bit messy. The original game was developed at Interplay Entertainment, and its first sequel was made by much of the same team after they moved to a different Interplay division called Black Isle Studios. Black Isle was working on a version of Fallout 3 - codenamed Van Buren - that was canceled in 2003, years before Bethesda picked up the rights to the series and made the Fallout 3 that we actually ended up playing.

For his part, Cain left Interplay after the release of Fallout 2, so his secret sequel wouldn't share any overlap with Van Buren or Bethesda's Fallout 3. "There are other people on the team who have their own ideas - I'm talking about the original team - of what should have been done in Fallout," Cain explained in the podcast. "That wasn't really explored in Fallout 2. Fallout 2 was done very quickly, so Fallout 3 probably would have explored a lot more of that."

While Cain won't talk about his secret design, he did offer a few hints about what direction he was