What Could Have Been: Abnett has revealed that he lost the text of one of the books and had to rewrite it at short notice... and in the process invented the character of Lijah Cuu on the spur of the moment.
There's an in-universe example in Blood Pact, where Tona tells Gaunt that Slaydo's choice between Macaroth and him as succeeding Warmaster was essentially a toss-up and he could have been the new Warmaster. Gaunt tells her to perish the thought.
Meaningful Name: First and Only. Abnett states in Black Library Classics reprint that he expected to return to comic scene after he's done with his first attempt at commisioned novel, but the series got off on its own.