a. Takuma Inoue - WBA
b. Ryosuke Nishida - IBF
c. Yoshiki Takei - WBO
So that is already a big company, and so he had to beat some of his fellow Japanese fighters first because he gets to the top dog of the Japanese boxers. With that, I do agree if he will go for Takuma and get his WBA belt as a start.
It just that it will never be easy, it would be better to persist for some time and improve things slowly, when Nakatani is truly ready then it could be the right time to move up and fight other fighters in the higher classes.
Of the three fighters you mentioned, I sure Takuma Inoue and Yoshiki Takei will be worthy opponents for Nakatani in the future, they will both show fierce fighting and I sure it won't be easy to finish faster or knockout.
After all, of course he will have this desire, to be able to achieve greater success in the future, the Nakatani era has just begun and he must really defend what he has got and pursue what he wants.