Fate/stay night

[Ryllharu]

btw, a sequel for Fate/Stay Night wouldn't be impossible... Saber destroyed the Holy Grail in the previous Grail War too (that's why the town was destroyed and Shirou was the only lucky one to survive... fate? ^^), yet it appeared again...

They explain this a lot better in the Fate route of the game.

Saber "broke" the Holy Grail in the previous war, when she was the servant of Shirou's father, the Angra Mainyu corrupted the grail in a much eariler War, and its essense spilled onto Kotomine and his servant Gilgamesh (an Archer) corrupting the two of them. However, Saber and Shirou together destroyed the Holy Grail outright at the end of this war.

so, it's not like the whole thing about the Holy Grail Wars is over now.
what I don't know is if Saber is still considered a useable Hero for the Holy Grail War...
She accepted what happened in her live and ordered Bedivere to return Excalibur to the Lady of the Lake and her life ended... so she doesn't desire the it anymore, right?..
(because of her spirit which fought in the holy grail wars and eventually became Shirou's servant)

So the next War might be with a different pool of heroes to choose from, but I don't know if it is possible for Saber to delete the contract she made with the Holy Grail or if her spirit really has to fight forever, even if she doesn't want the Holy Grail anymore.

Again, this was mentioned in the anime, albeit briefly, but better explained in the game (even with a flow chart!).

Saber, strictly speaking...isn't a spirit. Because of Arturia's obsession with finding the Holy Grail, as she lay dying, she is pulled into the Holy Grail wars across time by the power of the Grail. She's alive, and each time she returns to that spot, still dying, she takes back her memories of each war she participates back with her. That's why she remembers Shirou's father. Spirits don't retain any information from their participation in the Grail Wars. Each time they are summoned, they only remember what they took from their past lives. The only exception, of course, is Archer. It would be a spoiler to say why, as it was only alluded to in the anime.

With Saber finding the Holy Grail (even if she destroyed it) she fulfilled her only regret in life as she lay dying. She learned that she didn't need to obtain it, just finish the quest for it. She would not return again.

well...but this is IF there is a next Holy Grail War... for some reason everyone says that the grail is *really* destroyed this time... I can't find out why though... didn't Shirou's father do the same thing with the 4th Holy grail?

Kotomine explained this in the anime. They couldn't destroy it, it disappeared after Kiritsugu and Saber damaged it.

[Spoilers]

[spoilers]Spoiling from an anime point of view, but whatever, his identity was heavily implied by seeing Rin's jewel when he faded. Archer exists from an alternate reality in the future. He, unlike Saber, is not a fluid entity. He is a holy spirit from the future, and any events in his past are unchangable. It may seem that he is, but that is only because he participated in this set of events. What happened to him happened, he remains consistent through all three arcs of the game.

Two of the arcs he persists in saying, "It won't happen again," while in another, he very noticably shifts to "It is happening again." If you complete enough of the bad endings in that arc, you also learn exactly which one it is, as it says it plain as day.
[/spoilers]
You see the singular decision that crushes Shirou in that arc of the game.

You can't show Archer's origin without showing that arc. Shirou doesn't change into who will eventually become Archer until after a certain point in that arc.