Nature Chakra is 50% chakra boost to naruto,
he adds an equal amount of nature to balance his yin & yang chakra.
But ye he would need to fuse with someone to draw in Nature chakra for him.
Would be nice if he could use SM while in Rikudo Mode
1. Black
2. White
3. Nature
Giving his Rikudo mode a 50% chakra boost. which would be very bad arse.
Maybe the jutsu gets a boost of say 50% though i don't recall reading that exact value, but regarding the chakra, I'm pretty sure it's 1/3. Before the new seal it was 1/4 because of the Kyuubi if I recall. The point is to balance your yin, yang and nature chakras. Thus, they must exist in equal magnitudes.
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Either way, The amount of Kyuubi chakra he uses in Kyuubi mode is far greater than 50% of his normal amount. Thus, whichever, is correct, is dwarfed by the Kyuubi chakra, so it's kind of moot.
Yes, I agree that fusion of the two modes would be nice so long as it's not overpowered. 3-tomoe Sharingan and EMS are fused in a sense, so it might happen, but Kishi needs to be careful since we still haven't seen the limits of Kyuubi mode yet.
Peace