I appreciate the idea tbh. At first glance, it seems possible, feasible and somewhat balanced. But it has many faults in it when looked in detail:
-Like Lili pointed out, Genjutsu isn't a skill in which more chakra = stronger effects. Its not exactly like that. More chakra would indeed allow a greater hold of the genjutsu in a person but that can't be translated into the RP. Why? Releasing a genjutsu is done in relation to its rank. Allowing something to increase the grab of a genjutsu on someone means allowing something that by default increases the rank of genjutsu. Now, this has tons of issues. For one, genjutsu that are A-Rank most of the times wouldn't ever be approved if they were S-Rank as they would break the game. Same can't be said of Ninjutsu techniques for the most part. Also, any S-Rank genjutsu would become virtually unreleasable and unstoppable. You need to spend more chakra when using a high ranked genjutsu because of its details, its complexity. Using more chakra with Genjutsu is a byproduct of its complexity not a direct relation with the damage it does. When you mention the concept of Sharingan in that whole argument it became apparent you dismiss something quite crucial: Sharingan is a KG linked to Yin Release. All of the abilities shown from the Sharingan itself revolve around enforcing your will and making it take form. This form is lifeless, because its not real. Not tangible. But what you make others see is bound to your own will and imagination; its a product of it. Thats more or less the definition of Yin Release. Thats the reason why Sharingan Genjutsu is treated separately from normal Genjutsu. Because its powered by a KG that revolves around a core skill, Yin Release. Genjutsu itself is, however, simply sensory manipulation. Its creating false images and sensations on enemies. Nothing more. So overall, there is a disconnect between how a CM would translate into an increase in Genjutsu power or damage.
-CM is something devised by Oro using Jugo's dna. He creates a seal in the subject which contains jugo's DNA. When you release the seal partially or completely, the DNA begins to absorb Natural Energy in a rampant and incontrollable way. This leads to what can be seen as a pseudo or lesser version of a Sage Mode. A forced absorption and assimilation of Natural Energy. Now, Natural energy increases the potency of your chakra which translates into stronger Ninjutsu or Taijutsu by either augmenting one or the other. Natural energy itself seems to have no real effect in Genjutsu power or ability apart from potencially releasing you from one in some cases. Jiraya knew about Genjutsu and was able to use low forms of it. Yet, when in Sage mode, despite the enormous increase in his abilities, he was still not proficient enough with it to be of any use. The toads themselves, always in a state of sage mode, used a powerful Genjutsu but a Genjutsu with nothing different that other Genjutsu. An illusion. Strong, yes, but it was used by 2 individuals at once and we have no way of knowing if it was already strong on itself or not.
-Sasuke breaking from Itachi's Genjutsu had nothing to do with his CM. In the imaginary world he released his CM but not in reality. In reality, he fought his brothers illusion with his own visual prowess. It was hinted that old saying that a skilled man with a broom is more deadly than a weak man with a sword. Sasuke was simply more skilled with his own 3T to the point that he could break the MS grade illusion his brother used.
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Zetsu notes clearly that to his own eyes, they haven't moved. Nothing happens. All that we see before and after are within their own minds, in Genjutsu.
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You can see that in reality, once Sasuke releases himself from the illusion, he isn't transformed. CM was simply how his mind manifested itself in the illusion to release it.
-Adding something like that would, in the long run, cause tons of trouble. A CM is something you can add to a plettora of bios by default. Imagine that is added to a Kurama bio. Real Genjutsu that is stronger? We cannot forget the amount of Gen approved over the years that would become one hit knockouts.
-If you have a CM, you can already submit in the bio itself, the custom look of how you'll seem once you fully transform. But thats the limit of it. You cannot add more abilities than those already granted by the CM itself. You can only add a cosmetic feel to it. Nothing more.
Sorry but I don't agree with this.