So what might have been Elan's super secret plan?

[Yendor]

Yeah, well said, I concur.

I might even go so far as to say I think it's more likely Rich never actually came up with Elan's actual plan and just plans to reference Tarquin's defeat offhandedly in the denouement, than it is that he is planning to write a sequel book about how Elan's plan defeated Tarquin.

Fun idea: Tarquin's defeat (but not the plan leading up to it) is covered in a bonus strip. He goes to Hell and finds a copy of the book, which shows that not only is his demise not included in the main story, it takes up a single page.