Okay, I've never made a story before, so I'm just going to run into this brick wall, hahaha, here we go!
I don't own one piece Oda does womp womp.
So Luffy was running around trying to find Apple head and Bonnie when he found a cool-looking box and went "cooooool!!!" And then of course instead of trying to find Vega Punk and Bonnie, he opened it because luffy was luffy and inside was... an orb? Luffy thought to himself, he then picked it up and threw it away because he didn't know what it did, and as it hit the ground it went click... beeeeweeem... chirp. Luffy let out a little hmm as it shined blue, and luffy was absorbed into it. Meanwhile twelve years earlier...
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[You know what time it is] mountain bandit time. Luffy sees Himura walk into Makino's bar. Luffy was obviously confused by what he was seeing and then well Himura dumped a bottle of wine on shank's head and then and only then did Luffy realize that he was small and after a lot of hard painful thinking Luffy then realized that the lame mystery orb in the really cool box sent him back in time. Luffy then saw his devil fruit and thought to himself oh better eat that (ugh even worse than last time) He said to himself. Shank's looked over to ask Luffy what he meant and saw that Luffy had eaten the (hito hito no mi model Nika) he loudly as he could yelled out. LUFFY YOU IDIOT THAT WAS THE A DEVIL FRUIT YOU JUST ATE YOU MORON DO YOU REALIZE HAPPENS TO YOU IF YOU EAT ONE. And in true Luffy fashion he just replied oh yeah I can't swim any more right ? Wait how do you the hell do you know that Luffy. Oh Vega Punks weird orb sent me back in time. Then the whole bar went silent for a few moments ... ... ... WHAAAAAAAAAAAAT HAHAHAHAHAH NO WAY GREAT JOKE LUFFY. Luffy in retort yelled I'M NOT KIDDING WHY WOULD DO THAT. Ok prove it Luffy what is four times two. Uhhhh eight Luffy replied. HE REALLY IS FROM THE FUTURE THE WHOLE BAR ERUPTED while Lucky Roo even spit out his drink in shock. STOP BEING RUDE Luffy shouted. Then Shanks said all right all right you miiiiiight actually be from the future Luffy. Luffy then Thought on how to prove it further aaaaaand then immediately punched Shanks through the wall and the one behind that etc etc Luffy then appeared in front of him and said does this prove it shi shi shi with his signature smile on his face. Shank's utterly befuddled by what had just happened, Simply stared at Luffy in utter shock mostly because his face said he was Luffy but his strength said he was Rayleigh. Then Shanks just shook his head and said wow Luffy you are damn good pirate before letting out a giant roar of laughter. Luffy started laughing with him and then asked Shanks for his hat. Shanks then asked why Luffy would want his hat he just said that he was going to return it after he became the KING OF THE PIRATES. Shanks agreed and then promptly left the island after a good story talk with Luffy.
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Luffy then went on to beat Himura into a bloody pulp for two reasons A was to collect his bounty and B was because he was mean to Shanks, Then Garp came to take Luffy to Ace and Garp was SOOOOOO happy to see Luffy wasn't the criminal that had been caught but instead Luffy was starting his career as a marine at an early age and gave Luffy the eight million berries and then he said to Luffy so your listing to my advice and have decided to become a marine. But of course Luffy shouted HELL NO I'M GONNA BE THE KING OF THE PIRATES SHITTY OLD GRAMPS , Garp then tried to give Luffy the good ol fist of love but Luffy having observation haki dodged all of Garps attacks. Garp obviously confused at to what just happened asked Luffy who the hell had taught him haki, and Luffy without any thought immediately replied oh it was Mr Rayleigh in the future. Garp then stared at Luffy and went oh...ok then after a few moments Garp asked Luffy how he went backwards in time and Luffy explained while making Garp feel bad about what happened to Ace and after all that they went to dadan's.
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When they got to Dadan and the mountain bandits home Garp explained to Ace how Luffy his grandson was waaaaayyyyyy out of his league in terms of strength and how he was actually hoping he would train him which prompted Ace to try to hit Luffy with his pipe which in turn made Luffy use his conquers haki on Ace knocking him out instantaneously. Which also caused Garp to burst out laughing at the sight and then just leaving wishing Luffy good luck.