https://worstgen.alwaysdata.net/forum/threads/shanks-is-the-head-of-the-underworld.8947/
new headcanon is all of the red hair pirates panicked when luffy ate the fruit but it cant have been because of the curse, since luffy already couldnt swim. it had to have been because the fruit is important or they were going to sell it for shitloads of money to get a leg up as pirates. luffy ate it and also (double headcanon) showed conquerors haki before shanks found out, so he changes all his plans and leaves town for a long time (reporting losing the fruit? negotiating a new thing?)
then when shanks finally comes back, lucky pops a guy in the dome and they make a big impact right away, he tells luffy theyre friends, inspires him to be a pirate on purpose. higuma gets away and shanks decides hes taking it on himself, cuts his arm off, goes "give this to the boss and tell him we lost the kid. im taking responsibility for this, hes got to stay alive" or whatever and jumps in to save luffy and make sure he's strong before he goes out in the world. then after a couple more years shanks is the underworld boss and not just a guy trafficking devil fruits.