One Piece: The Justice of the Supreme Ruler

Because the protagonist defended Garp, internet trolls called him a Garp fanboy. How could he put up with being flamed? He fired right back, only to transmigrate into Sengoku just after his great battle with the Golden Lion.

As a transmigrator, he naturally has a System, but there's a catch: it won't work properly until he retires at the age of sixty.