Having looked up Sekigahara at imdb.com I take it the Mifune de nos jours must be Koji Yakusho, who plays Tokugawa. He certainly has some chanbara pedigree, having appeared in 13 Assassins.
That's him! Like Mifune, Yakusho's played General Anami and Admiral Yamamoto and is considered something of a must for prestige productions.