Though Liam Neeson only got to star as this Jedi Master in one film, the character became immensely popular; his personality perfectly captured the nobility of the Jedi Order, but was just rogue enough to serve as an underdog. He was also the only character to recognize that little Anakin Skywalker was the Chosen One, an accurate presumption that eventually led to the downfall of the Dark Side.
But Qui-Gon died at the end of The Phantom Menace, leaving his apprentice, Obi-Wan Kenobi, to train the young Skywalker. And we all know how things went from there, right?
Hints of Qui-Gon’s past were made throughout the prequels, including the fact that he was trained by Count Dooku, so there would definitely be a lot of material to make a movie out of. A spinoff surrounding Qui-Gon Jinn’s past would be most effective if it focused on the Jedi training process. Throw in a couple missions and lightsaber battles, and you have the formula for a film that’d surely be better than any of the three prequels.