The War Never Stops
CANON INFO: For this site, Unlimited Blade Works and Arcueids True End are what is considered canon for the plot. The other routes didn't happen. That means: Shirou has his contract with Rin and his Reality Marble with the ability to turn himself into one gigantic magic circuit to create swords from his mind, and Arcueid disappeared after the events with Roa, leaving Shiki behind. Also, the bad fanfic - aka F/H/A - did happen, but all Servants appearing in it and the people were nothing but the dream of Avenger. Meaning Saber, Rider, Lancer, Gilgamesh etc have all disappeared after the Holy Grail War. Sakura and Shinji both disappeared after recovering from the war, possibly to hide from Zouken's wrath.
When the grail was destroyed, the war ended - seemingly forever. Leaving for the Mage Association shortly afterwards with Rin, to become a better Magus - and able to destroy the Magic Association from within - Emiya Shirou left his home town behind.
At the same time, after losing Arcueid, Tohno Shiki seemingly returned to his 'normal' life in the middle of the big screwed up family called the Tohno clan. He somehow managed to survive both Ciel and highschool without too many problems, but his heart forever belonged to the vampire that disappeared.
And so, the years passed. With the guidance of his now-girlfriend Tohsaka Rin, Emiya Shirou mastered the use of his Reality Marble, turning into the Magic Circuit himself instead of using the few that he had inside of his body. Tohno Shiki seemingly lived a normal life, but 'strange eyes attract strange beings' - he couldn't help encountering supernatural beings and having to fight from time to time.
But now, the pieces are moving again. Nobody knows what caused it - but the biggest nightmares of the world returned. The Serpent was back - though entirely different now, with a new persona and abilities, Roa was back. But not only the vampire, but also the great grail returned - something that should've been impossible.
What exactly caused the return is not entirely assured. Maybe it was the evil will of the Angra Mainyu inside, or that the limitless energy couldn't be contained. However, as the 6th Heaven's Feel assaulted Shirous home town after exactly ten years, he and Rin instantly traveled back - only to arrive too late.
Someone reached the Holy Grail in the most swift and brutal war so far, a thoroughly evil Master and a tricked Servant being the last ones standing... And a wish for eternal war breaking loose.
Now, the grail has gone out of control. Summoning a large amount of weaker Servants, sometimes even without Masters, even normal humans can now truely become the Master of a Servant by chance, and the Church is on the move to contain this phenomenon. In addition, the dark Magus that caused this calamity is still around, now allied with a Caster of unheard prowess...
Now is the time of change. The future is unsteady, and more and more future heroic spirits are appearing. Take up arms, heroes of old. Magi of the new age, surpass your limits. Heroes of the unseen futures, change fate itself. Creatures of the night, now is your time. Destiny is in your hands, in the world of...
Into the world of Fate/Endless.