February 11, 1998
Xenogears features a poster of Tifa from Final Fantasy VII.7
Final Fantasy VII
January 31, 1997
Final Fantasy 7 features a minor reference to Xenogears.
Theatrhythm Final Fantasy
Curtain Call features a downloadable track from Xenogears;
For more information see:Theatrhythm music.
World of Final Fantasy
While World of Final Fantasy feature a mirage called XG, based on the Xenogears.
Square Enix has the rights for both series.