The phenomenally successful TV series was adapted for the box office in 1998, and a sequel followed a decade later (08).
The X Files' stars David Duchovny and Gillian Anderson have both insisted they would love to play FBI special agents Fox Mulder and Dana Scully again, and now Spotnitz - who wrote for the series and movie versions - wants the franchise to be a trilogy.
He tells Sciencefiction.com, "There is a very active and relentless fan campaign for a last movie. I do feel like it would be a terrible shame if that didn't happen.
"It feels wrong not to give it an ending around the alien colonisation of Earth... I have a clear idea of how it would go and I've been talking to (creator) Chris Carter about it for a long time.
"I'll keep banging the drum whenever I meet the Fox (studio) guys."