Future Majin Buu was sealed a long time ago, with Future Babidi plotting to unseal him, however due to Future Trunks, Future Dabura fell in battle and Future Babidi was killed—preventing Future Majin Buu's revival.
Video game stories
Shin Budokai - Another Road
As with his alternate timeline counterpart, Future Majin Buu is also resurrected by the energy of the battle between Majin Vegeta and Goku. Vegeta confronts Buu, but Future Majin Buu defeats him. Future Babidi tries to send in Future Buu, but Future Buu refuses to listen to Future Babidi. When Future Babidi sees that he has been killed by Buu in Piccolo's memories and remembers seeing Namekian Dragon Balls in Piccolo's mind, Future Babidi leaves Future Buu in Future Dabura's care and goes off to New Namek to use the Dragon Balls of New Namek to place Future Majin Buu under his control in order to change his fate.
Future Buu later meets Future Mr. Satan. Mr. Satan gives Future Majin Buu a pebble for a present, and they start playing together, however Future Dabura sees Future Buu playing with Future Mr. Satan, and he launches an attack at Future Mr. Satan. This causes Future Majin Buu to become more enraged than he has ever been, and the steam he releases out of the holes on his body coalesce and form a new Buu.
Dragon Ball Heroes trailer
What appears to be Future Majin Buu is in the second promotional trailer for Dragon Ball Heroes. This Majin Buu is in a city and turns people into candy, until Future Trunks and the Saiyan Hero Beat appear and fight him.
Dragon Ball Xenoverse 2
Future Majin Buu is stated to be stronger than the original timeline's Majin Buu, due to him being given more energy while he was still inside his Sealed Ball.