Chapter one HP/ATLA/DB

Goku, or Zuko, or, once upon a time, Harry Potter wasn't sure where to start when thinking over his past and present lives....
Perhaps the fact that that in his first life he became the Master of Death; a nigh-absolute immortal with regenerative powers and limitless potential?
Well in his first life, he began to reach his full magical potential and grew immensely in power, but continued growing in his second and his present third life.
In his second life, Goku was named Zuko, a Prince, then Crown Prince, exile, Crown Prince, and then, finally, the Fire Lord. In his second life, he was naturally blessed with reasonably large chi reserves and pushed himself in training for most of his life to enlarge them even further... it was after hellish training and battling the Avatar in the Avatar State that the then Zuko unlocked more of his potential and latent power. After several battles, Zuko was just as powerful as the Avatar, but still Zuko trained for over a thousand years until an extinction level event
The results carried over to his current life. Chi was called Ki here and Goku had a lot of it. Enough to give him a hidden power level of ten million at birth. It wasn't until Goku injured himself at age four that not only was his hidden power unlocked, but also Goku's true self, an immortal Master of Death was released. Goku was happy and even had a father figure to care for....
Until he transformed into a big, dumb, ugly, and uncontrolled were-monkey and killed his Grandpa Gohan. Yeah, that soured his mood and the pain from cutting off the tail that he did in atonement made it worse. Goku cried for the first time in centuries of his life spread over two lifetimes.
When Goku finally mustered up the will to continue on, he decided to use his immense intellect from two previous lifetimes and create technological aids for training... like Gravity Chambers and Time Compression Chambers and then atone for his crime.
Goku spent eight years training, gaining power and skill, and building a stable foundation that would allow him to eventually become the strongest being in the universe... and nearly doubling his power was a great start.
Soon after Goku had doubled his power, his castle was visited by a young, beautiful, blue haired girl that desired one of the few mementos Goku had left that reminded him of his Grandpa Gohan. Despite Bulma's attempts to deceive him, Goku did join her on her journey and quickly began to fall for the beautiful and highly intelligent young lady. Bulma wouldn't be interested until he was legal, but Goku was persistent.
Goku dominated any fights he engaged in and when the Dragon Balls began to scatter, he caught the Four-Star Dragon Ball.
Goku spent the next six years alternatively training, fighting, saving the world, making money, and growing immensely in power by lowering his power enough to allow enemies to injure him so that he could recover and imprint on it.
It was in Age 756 that Goku not only defeated Piccolo Jr. and won his third tournament, but Goku finally convinced Bulma to marry him. It was difficult, but them using artificial wombs to spare her body of damage from childbirth was a major selling point.
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Goku was proud of himself. He was extremely powerful, a Super Saiyan, a devoted father of six, a happily married husband, rich, influential, and had the potential to become the most powerful being in the multiverse. Goku had unfortunately been warned that it was time for him to fulfill his purpose here. His brother in this life was going to recruit him into servitude to a tyrant... a tyrant that Goku would likely have to defeat.