That's why this is a fanfiction. There is no insult if he gives proper relationships to the different powers and their balance. And you talk like there are no ninjas or ninjutsu in the One Piece world. If your words are 'putting powers from the worlds in One Piece is an insult', then One Piece is insulting itself. First it was just devil fruits. Then came different races. Mermen, beastmen, oni, giants, lunarians, and the powers haki, rokushiki, ninja arts, brrr dials, robots, cyborgs. Then there are souls, ghosts...One can add anything to it and it still won't be a problem.

Yeah, haki low-key felt like it can out of nowhere, and now it's the most powerful ability to have in One Piece.

rather than insult it is more like a "what if story" which is the core of fan fiction, although some fail to incorporate two different power system as it would mess up the balance of abillities in which most fan fic authors just nerf the said abillities. nerfing of the abilities that is from a manga or anime is a lot more insulting than putting abillities from different anime to one piece as if it has no abillities of its own. frankly paramecia is the most op devil fruit a isekaid person from a modern world who knew intermediate or advance science could get as the devil fruit abillities is based on the imagination of the user. just take buggy's devil fruit for example if he just know the concept of splitting cells and its op application with the use of his devil fruit he would be op individual or if he goes further and have knowledge of atoms buggy would be able to disintegrate himself for a moment and evade all kinds of attacks and reform again heck he will be able to reform or gather his body together inside of his enemies body killing them gruesomely. as devil fruit is based of element for logia, concept for paramecia, and creatures for zoan. which speaks for the abillities that is equal of other animes its just that the fan fic author have a different idea and preference.