Imagination is an umbrella term that includes headcanon, but headcanon has a much more narrow and communicative meaning to do specifically with interpreting missing or ambiguous details in established fictional universes by fans or other people who aren't part of the creative team for that universe. Using the word that best fits the idea you're trying to communicate is good, actually. Also, while English does have real words what makes a word real is nothing more than whether when one English speaker uses is can they reasonably expect other English speakers to understand it. Given that everyone who read this thread had no problem understanding the word "headcanon" (including you, despite your protestations, as you were able to define a synonym) that means it's a Real English Word(tm)(c)(r).