Should I make the last samurai breather in next series?
Yes. And also, don't write "Last Samurai", we want "MORE SAMURAI".
Plase spam the whole story with more of those Samurai, doesn't matter if they're just knockoff or weaker than of those Samurai you've already added.
By the way, let me warn something. It might be copyrighted due to adding Demon Slayer's breathing style but as it is about the last samurai breather, he would use Wind Breathing but different styles.
Just "rewrite it" into your own style. As long you don't use the exact copyrighted naming, you're free to copy the mechanism of those breathing sword techniques.
You can even copy "bankai release" from bleach and "Amakakeru Ryū no Hirameki (Heaven's Flash Dragon Flash)" from Samurai X if you wish, as long as you only copy the 'mechanism' and not the name.
If you want to copy Bleach's bankai, just say that your samurai is using a Mitsuru transforming sword technique that will evole his sword into its final, ultimate form, compressing spiritual energy for massive power boosts in speed, strength, and special abilities.
Congratulations, you just copied Ichigo's bankai and your samurai character no #7 are now using them.
If you write in your description Flash Tiger sword technique, in which "The ultimate secret sword technique, involving a powerful upward slash that creates a vacuum, allowing for a second, stronger blow upon the opponent's collapse or recoil."
You have sucessfully copy "Heaven's Flash Dragon Flash" without repercussion. Congratulation, you just copied kenshin.
People can accuse you of copying as they like. But you just need to say "ahh its not the same! It's different in this aspect and that regard!" And people will be forced to accept that as canon.
Want to copy breathing technique from demon slayer?
Just say that "Breathing Styles" are swordsmanship styles that makes use of an esoteric breathing technique known as "Sword Breathing Technique", and are practiced and taught by members of the "Samurai Clans" to be used in warfare.
Many story have interchangeable format like xianxia in which every story left and right are using qi for cultivation manual, with some of them renaming the technique left and right.
This is the same. If you read many other fantasy Samurai stories, they usually copy thing from one another.
Have fun!