“The plot itself does exist”
Final Fantasy X was the first mainline instalment to get a sequel, and its makers have hinted a return to the world one day could happen with Final Fantasy X-3.
In a recent interview with Famitsu (translated by RPGSite), character designer Tetsuya Nomura states the story for a potential sequel is on the shelf and “If we are to make 10-3, we somehow have a synopsis written by [Final Fantasy 10 writer Kazushige Nojima]”, and that "It's sleeping right now, but the plot itself does exist."
Nomura says its not impossible for X-3 to be made, but the Square Enix team is busy working on the Final Fantasy VII Remake series.
Basically the concept of X-3 is still extremely early and it could take another decade for the game to come into fruition. It is unknown how many parts will comprise the VII Remake series before Square ever gets to work on a X-3.