Once Again has its own tvtropes page. You can find the link in my profile. Feel free to add any additional tropes if you're familiar with them.
4. Setting sail (again)
Shanks, despite planning otherwise previously, stayed on the island for another month because of Luffy's pleading. He and Ben taught Luffy a few things, such as the basics of swordplay (Shanks gave Luffy one of his swords, one of the Skilful Grade Meito (Named Sword), Ashita no Kanashimi (Tomorrow's sorrow), a relatively small katana with a black hilt and sheath with a green line coming from the middle of the hilt to the end of the sheath (on both sides) and a silver blade with a slighty purple-ish glow.
Afterward, when Shanks left, things proceeded as they were supposed to, with meeting Dadan and Ace and Sabo and everything that was supposed to happen after that. Luffy hid his haki, his strength, as well as his knowledge from his family, so he appeared the weak little brother again. He actually enjoyed the experience of having two big brothers looking after his supposed weak self. Also, he thought it would be priceless to kick Ace's Logia ass when they next met and then reveal to them that he wasn't weak after all. When Sabo was supposed to die and his boat was shot by the World Noble, Luffy, with his super speed knocked out Sabo and made it appear as if he by some miracle survived and was washed up to the shore alive. He made himself cry by remembering the deaths of his nakama and friends in the previous life. Truly, he became quite devilish and very different from his first, less intelligent self. But, in essence he was the same. He still cried quite a bit, though now for different, more serious reasons. He still loved meat and adventure and was still horrifyingly careless at times.
Then Ace and Sabo set sail, this time together, as co-captains. Sabo didn't have the ambition that Ace had (which was to defeat Whitebeard), he only wanted to be free and Ace wanted to protect Sabo. Also, they wanted to take Luffy with them, but he flatly refused them each time, stating that he will become Pirate King on his own.
Afterward, Luffy could finally start to train without being worried about being found out. He took back his small katana from Makino, who was keeping it safe and remembered his short, but useful training with Shanks. To his relief, he found out that his haki got no weaker from the discontinued use, but he wanted to become stronger. By seventeen, he his chest became quite a bit more toned, now rivaling his future self that merged with him.
At the same day as before, Luffy set out to the sea. Once again on a small boat and although it seemed almost the same, it clearly was not. Luffy was different. He had his small Meito strapped to his waist and he wore different clothing. He now wore what his future self wore, a red open sleeved shirt revealing his mighty chest and scar, his hat was now secured to his neck by a string and he had black shorts with pockets in which he had quite a bit of money from his days at Grey Terminal, when he hunted small time criminals for money as well as a small box containing some important items. Hidden by his sleeve, he had a small tattoo on his right shoulder now. It read 'ASL'. 'A' was yellow with a red border, representing fire, 'S' was light blue with a black border representing his hat and 'L' was red with a pink border representing him. The villagers waved at him as the left to shake the world. Again.
Yes, Luffy saved Sabo! Even if it were to be revealed that he is still alive in the future, Luffy doesn't know that and thinks he died then. If you're mad that I didn't go into much detail it's because I'm frankly not very interested in the childhood saga. I may do some flashbacks someday, but I'm much more interested in the main story, which will begin in the next chapter.
Why did Luffy cry? He's merged with his future self, but he's still a kid.
Why did Sabo join Ace? I don't know much about Sabo, but I figure he just wants to be free, so he joined Ace, because he was being annoying and overprotective about him and he didn't really have any real desire to be the pirate king, so he figured he might as well make his brother sleep easier by joining him. Luffy refused Ace's invitation for obvious reasons.
The name of the katana? Ashita no Kanashimi (Tomorrow's sorrow) kinda fits Luffy, doesn't it? But I chose it, because it sounded cool and only after that I realized how well it fit. First name (Tomorrow's sorrow was second) I thought of was also random. It was Tamahen (Reason), but that doesn't fit at all, so I discarded it.