Rule suggestion: Spiritual nonequivalence
In some outside forums there's a commonly known and accepted rule known as "Energy equivalence" which--though a misnomer--basically suggested that all spiritual powers (chakra, chi, magic, reiatsu, etc.) should be interchangeable. The main benefit of this is that it's easy to understand, characters will be more versatile and it's "fair". However, the problem is. It doesn't work.
If you were to argue with your friend that Uchiha Itachi from Naruto would defeat Uryū Ishida from Bleach, through genjutsu and he in turn would bring up that the mechanics of genjutsu are designed that it could only affect characters with a chakra-based nervous system that argument is efficiently refuted.
Of course there are even more absurd examples such as: "Neji Hyuga would cut off Kenpachi Zaraki's chakra system."
To try to counter by saying that it's unfair is to commit the is-ought problem fallacy.
Does this mean that genjutsu is worthless? No. Characters with chakra-based nervous systems--namely characters from Naruto--would still be affected with it. This would somewhat abstract power-scaling, but if you're well versed with said fictions it shouldn't be more of a problem than understanding the concept of rock-paper-scissors. Or if preferred:
[list][*]Bulbasaur > Squirtle
[*]Squirtle > Charmander
[*]Charmander > Bulbasaur[/list]
But there are more problem than that with "Energy equivalence", since the reasoning doesn't even hold within certain works of fiction. Mainly fictions with more than one type of spiritual power, for instance Bleach as of recent.
If the Koutotsu (arresting projection) is immune--or at least heavily resistant--to reiatsu no spiritual force of the presented caliber should be able to hard or even faze it. The outcome proved to differ.
The benefits of this system would be more accurate and detailed outcomes since more variables are taken into account.
Please contribute with arguments and votes.