I'm seeing one huge problem here. For an RP based around 'character interactions and building', your drastically limiting character build options for the sake of pvp 'balance' and making everything super watered down and boring in the process.
I mean, sure if you just want a hunter x hunter sorta thing going, then just say that. But then nein can be used to an extent that is very close if not exactly magic. (Greed island) or borderline alchemy (Chimera Ants) so it seems like that isn't even happening here.
I dunno, just seems like the initial vision, then your refinement of that vision, are two totally different ends of the spectrum. Aren't shonen anime supposed to be fun and over the top? Fist of the North Star being a great early example. Or bobobo, if you want a shonen parody. (I miss that one.)
What I really want is something that's a good medium. Something that's mostly fun and bombastic, but not so much that it can get dumb. If I ever implied that I wanted something other than that, then I apologize for misleading you. This is why I'm willing to bend on the item creation somewhat
If your super worried about PvP being unbalanced, you can always just make death an inconvenience instead of a certainty with dragonballs or something. Its not going to happen very often if your trying character development stuff anyway.
I don't really like that idea for the same reason I don't like it in those stories, it makes death meaningless and feel like there's barely any stakes at all