Season 5 Part 1 slipped badly after "White Noise", with "Nocturnals" being uploaded bi-yearly at one point, but after it was finished garfieldodie uploaded Part 2 and explained that real life got in the way, and that both the rest of Season 5 Part 2 and the rewritten version of Calvin and Hobbes II: Lost at Sea are in development.
What Could Have Been: "Temporal Delay" was originally named "Terror from the Year 2011", then "2012", before adopting its current name for two reasons: the title would become dated very quickly no matter what year was chosen, and there's isn't really a "terror" in the plot of the episode.
The Season 5 bonus chapter is filled with this. See this page for all the unused episode ideas.
Word of God: The Season 5 bonus chapter provides us with a lot of this.