Dude, please put a big limiter to other world abilities, they make him way too OP. I link this kind of story and will be with you is this journey. If you think about dropping the story just don’t do it without telling us.
Dude... have you seen one piece at all!!? Gum-Gum is like one of the weakest paramecia type devil fruits. Luffy just brought out more of its potential and uses it recklessly. I bet when he finally becomes pirate king, he’ll be dying like Roger.
If you get him a sword give him narukami ( Toudou Touka sword from chivalry of the failed knight) and Touka sword style, it would fit well with his magic and her raikiri is awesome.