Nasu 'verse/Earth/Reverse Side of the World

BODY OF BLADES, AND HEART OF SWORD Rin and Shirou have somehow found themselves thrown back in time into the bodies of their younger selves. Given a second chance at the 5th Holy Grail War, the decide to rig the game as much as they can. Using Avalon as a proper catalyst this time, Rin summons Saber, the King of Knights. But with Saber already summoned, the Grail falls back on blood to call forth the servant most suited to Shirou. Shirou's body is made of blades; it should be no surprise his ancestor was named ‘Heart of Sword’. Or: Rin summons Saber, so who is left for Shirou? No, not Archer (luckily). He's already looking after Sakura. But blood calls to blood, and Shirou finds himself face to face with the Hitokiri Battousai. It looks like assassins runs in his blood along with the red hair. -- Includes time travel, a holy grail war doomed to fail, a bunch of Heroic Spirits determined to protect the world, a sword style created by a Divine Dragon for the explicit purpose of monster slaying, a possibly immortal swordsman, a family so troublesome even the mage's association don't bother them, and swords. Lots of swords.