Dragon Ball universe:
Vegito the warrior born by the fusion of the two strongest saiyans was fighting with the terror of the universe Majin Buu. As the fighting proceeds Vegito pushes Majin Buu towards the edge. In his desperate attempt to strike the saiyan Buu release a very powerful blast towards him and Vegito aslo retaliates with a big bang kamehameha. As the two blasts collided the result explosion started to weaken the dimensional wall. Vegito pushes further overpowering Buu's blast. As the blast was about to make contact Buu shielded himself with his ki but the blast causes him to go through the dimensional wall.
After defeat of Buu Goku and Vegeta uses dragon balls to bring everyone back to life completely unaware that Buu is still alive.
Justice League universe:
Buu ends up in another universe weakened by Vegito's attack. Buu woke up to find himself floating in the darkness of space and thought he was sent far away from earth by that attack. For the first time Buu realises in order to gain actual strength he needs to train and absorbing other people won't help him win against likes of Vegito who harness the true power they gain by putting their very life at stake to be the best in their battles. Buu remember Vegito going inside his body and coming out with Gohan, Goten, Trunks, and Piccolo. Buu decides to train on a uninhabited planet and return to Earth for his revenge against the saiyans.
Author's note:
I know things are a little different than the original Dragon Ball Z story but I took some liberty as it is my story.
In the next chapter it will be interesting to see when Buu finish his training and came back for his revenge but find some costumed superheroes as protector of Earth instead of the Z warriors. Will Justice league be able to defend Earth from the unbelievable power of Majin Buu.
