If I can chime in; I have some ideas about how to make it more balanced.
Because it’s the Rinnegan, of course there are some things more worrying than others, but I’d like to start at the basics; characters on creation, and permissions granted to them and the like. This is also my reservation for the deva path if they get allowed.
General SAs should require Chakra Supply and a slot for the Rinnegan - although I personally believe if someone obtains another rinnegan on top of their own, it shouldn’t cost another slot, but add onto their pre-existing one. They could also have a spot in the Special Abilities/Weapons index rather than be their own reserved position ala Jin/Sages, helping further limit them.
Having a cross between the Nukenin and Jinchuriki rules might be the way to go - forcibly making any Path DE’d, and require progression similar to that of a jin in order to gain more access to their path’s abilities. Some of them might be more gradual, such as the Animal Path getting more summons, or the Asura Path getting more equipment. Others would be more weighted, such as the Human and Naraka path only really having one technique. The Deva and Preta would be more awkward to measure out, but it could easily be that certain technique levels can only be acquired after training X threads or X amount of EXP.
On the grounds of EXP, let’s move onto purchases. Because none or the paths would be allowed clans or the like (duh), the path techs could be weighted as if they were neither of the user’s specializations, even if they might be - on account of the eyes being a weird hyper genetic deformity. "But Snake,” you might ask, "can’t they just begin with path techniques anyway?” Sure can, but let’s say you limit any Path to only starting at Chunin or lower, and that concern mostly evaporates.
From there on, concerns of "Well the animal path only needs kuchiyose to be useful” and "the asura path is too powerful from equipment” to "the preta path can absorb a ton of chakra anyway!” Rise up as issues. From there, I have some contributions.
Break the Asura Path’s equipment down into a few items that can be used normally, and then add jutsu that transform/enhance them. For the Animal, make each summon require its own Jutsu, on top of the Summoning Enhancement thing; probably with multiple versions for ranks.
These are just some rough ideas to contribute; if yall decide to go this direction.
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