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Dragon god slaying magic?
To complement my comment, you could also explore Sulong (One Piece) and the 8 gates. One more thing, the HunterXHunter universe is very vast and you could take that into consideration for a future fanfic, and if you're interested and looking for material, I highly recommend using the 1999 version as a base.
It's a difficult choice; the options bring so much potential, but if you have to choose, I would say go for Douluo Dalu (Soul Land), I find the work very interesting and with a lot of potential depending on the Martial Spirit and the Skill System, taking into account that you can be inspired by skills from other worlds such as shunpo, rokushiki, rasengan, shundo, seimei kikan and much more, and even without a direct connection with the original protagonist group, depending on the time/year within the story, many things change.