Which would it be, Marvel has done it before similar in the 70s with Godzilla, what if they get the license of the manga publishing company like Shuiesha or Kodansha. Take a license from a manga creator and use those characters and integrate them into the Marvel Universe. What would you like to be. Of course their backstory has to reworked a bit there to make sense how they exist with the Marvel characters. I always picture Sailor Moon as I noticed that's the franchise that often get crossovered the most in fan fiction and Jordan D White love the series as he has a podcast dedicated to it. if you take Dragonball z, you are gonna nerf the DB characters hard, My Hero Academia maybe(of course you have to rule out the premise of the story how almost all of humanity has quirks which wont work in the MU with mutants, Inhumans, Eternals, Superhumans and most of humanity). I used to fantasized this sorta thing. Back in my drawing days I even drew a Age of Apocalypse version of Sailor Venus with a tragic backstory which I'm actually quite embarrassed about.