(comment down if you like where the story is going, if not then please tell if I am doing well. I am re watching one piece to make sure my facts are correct.)
Calm Belt, Dog-Head Warship
You can see a man and a child fighting each other on the deck of the ship. The man was hitting the kid in front of him but whenever he touched him, he burst into in black flames. The man is Bogard and the kid is Kaen. It's been 6 years old since Kaen stepped on the boat and his entire life changed. He thought his training regime was the Saitama workout but Garp was a slave driver. When he finished the regime he got from Garp for marine training recruits, he asked for a harder one. At that time, he made a mistake he shouldn't have when he mentioned training and Garp's glinted. It made Kaen remember this old man he played and made fun of was strong to go toe toe with those who stood at the peak in this world. When he was ordered to do the new and improved regime he thought he was going to die. He had 400 push-ups, crunches, sit-ups, squats along with 20km. When he was able to finish the regime and not feel he was about to die which was 2 months later, Garp put weights on him. First it was 20 pounds, then it started increasing by 5 every time, he got used to it. Due to this training, he was able to carry 200 pounds and still be able to do this workout. He worked on his devil fruit when had free time from his physical training with Garp and able to recreate a few fire ninjutsu. He recreated "Fire Dragon", Fox Fire and even created a his new signature move, Dragon of the Darkness Flame. He even learned swordsmanship from Bogart who was pretty good. It turned out that Bogard was as strong as a Rear-Admiral but had the rank of Commodore since he did not have enough merits yet.
"You are getting strong, Kaen. You are quite natural at wielding a sword and already mastered Soru." Bogard praises him. Bogard knew that this kid is strong enough to match a Commodore in terms of strength and can even fight a Rear-Admiral for a bit.
"It's all thanks to your efforts and my training Bogard-sensei." Kaen replies being humble making Bogard nod at his action. What is the use of having talent if you don't practice hard enough. Many geniuses like the current admirals who after reaching the admiral rank do not improve at all. This is due to arrogance because not many pirates are able to kill or even match them.
Kaen remembers when they arrived at a island to train. Garp made him hunt a giant tiger but told him to not get cocky. After all, animals here are able to use a bit of haki making them perfect training partners for any devil fruit especially Logia type. Garp even warned him run in case he encounters a Giant Bear since it would be hard for a Commodore to beat him. Kaen was in his thoughts and pretended to listen to him. He went into the forest to hunt the giant tiger for food and was surprised to see him running towards him. Kaen laughed at his luck was so good but failed to see the terrified expression on the tiger's expression. Kaen quickly ran up to the tiger and inhaled a lot of the air. Then in a steady stream released fire from his mouth turning it into a fireball. The tiger who isn't looking at the fireball is hit with it causing the tiger to cry out in pain. Kaen is happy at this but suddenly Kaen's eyes widen at the scene in front of him. A giant bear was leading a 10 bigger than normal wolves with him. When the giant bear charged at him, Kaen changed into his Logia state to avoid the damage but failed. He got sent through multiple trees before he stabilized himself. Kaen feeling the pain is reminded that he is not playing any games but he is living in this dangerous world. He must be careful or will die again. Kaen's eyes shone and harden as he unconsciously releases a pressure making the wolves faint.
The giant bear seeing his followers get defeated so quickly, roars in anger and a hint of fear as he charges toward Kaen. Kaen seeing this counters with "Great Fireball" and it burns the giant bear a bit. Kaen seeing the damage, uses a stronger attack he recreated from naruto" Fox Fire". It creates 6 orbs similar to the one used by Pakura of the Scorch release allowing to him to either defend or attack. Instead of launching it, he combined all of them allowing him create a bigger orb similar to "Cruel Sun" but a 1/3 size of it. The problem was this move took a lot of his stamina making him unable to fight after this. Kaen launched the move as he watched the black flames engulf the bear. Kaen smiles at his but his expression changes to disbelief when the bear is still able to move. Kaen is on the verge on unconsciousness when the bear's paw starts to descend on his body. Kaen closes his eyes as he waits for his death.
"Sorry Grandpa Garp and Sengoku. I won't be able to become a proud marine you wanted. Sorry Uncle Kuzan, we won't have the fight when I become strong enough." Kaen whispers as he finally loses his consciousness as a shadow comes and slams a punch in the bear's head. The shadow's punched turned it into mush and creates a mini-crater on the ground.
"You fool, you should listened to me." The shadow reveals to be Garp, he saw his grandson was not fully listening to him. So he followed to see what would happen. He was shocked at the strength of his grandson displayed at such a young age. "He will turn into a bonafide monster when he truly grows up."
Kaen slowly wakes up in wooden cabin he lived in seeing his body bandaged a bit. Kaen was wondering what how he got here when suddenly a fist hits him the head.
"Owww Ouch, what the hell gramps? What the hell was that for?" Kaen glares as he looks up at the perpetrator but wilts down when he sees his grandpa glare at him.
"That was for not listening to me brat, you could have died." Garp says in angry tone as he harshly scolds him. Kaen looks down and clenches the sheet hard making Garp soften up a bit. Kaen knew he was weak in this world. He lost to animal and there many who are far stronger than that animal.
"Grandpa, I want you to train me the hardest you can". Kaen says as his eyes shine with determination. Garp looks at him for second and then smiles widely.
"Alright brat, I will train you to become the strongest marine stronger than your uncle and both grandpa. I will even teach you one move of Rokushiki for your strength but the rest will learnt with merit points or in the elite recruit taught. He remembers when it was taught by Z but defected when he lost his arm and his students were almost slaughtered. He along with his 2 other students went and created "Neo-Marines".
Kaen was interrupted by his thoughts by Garp who was smiling at him.
"Have we arrive there, Grandpa Garp. Can't wait to meet them nut we should stay for 2 weeks or month. We have to arrive or otherwise Grandpa Sengoku will throw a fit." Kaen advises Garp but he just laughs and dismisses the advice. Kaen was excited to him since he should be meeting the 3 characters in One Piece