Some legends refer to King Arthur as the Once and Future King. His grave marker supposedly said "Here lies Arthur, the once and future king."
taken from
http://en.wikipedia.org/wiki/The_Once_and_Future_King
Found an interesting summarized version of the legend of King Arthur from PBS
http://www.pbs.org/mythsandheroes/my...ur_arthur.html
Arthur is either dead but will revive at some later point or is gravely wounded but not dead and hoping to be healed at Avalon from mortal wounds, the legends vary.
I don't think it is much of a further stretch to say that Saber could wake up and emerge from Avalon many centuries later in Shirou's time, maybe some time after the events of the last episode. The ending of Fate/Stay Night is open to interpretation. Basically take your pick on what happens afterward.