
And spoilers follow.
Right, marathoned up until episode 19 of Fate and it gets pretty awesome. Very glad I decided to keep watching instead of dropping this. It started off pretty slow and sedately in the beginning but began to pick up speed right around episode 15 or 16. Shirou seems to be more foxused rather than the undecided guy he was before, he also seems to acknowledge Saber more now (Go Shirou!). To be honest, the only real reason I'm watching fate currently are for the action scenes. C'mon, admit it, Saber fighting is just hawt :P.
Also, Sakura in that black…whateverthatis…is simply too…what's the word…eye-grabbing to ignore. I know I simply stared at her in disbelief when I first saw her in it.
Oh and more importantly, Archer is most definitely, no doubts about it, Shirou. This may be a stale theory for those of you who've already passed these episodes, theories, but stick with me. They both materialise weapons the same way, with the whole grid and 'fleshing-out' sequence. Also, they fight the same way. Could Archer be Shirou from the future? If so, how the heck does his hair go white and his eye colour change?!
Keeping that line of thought, onto the OP.
I love it. I simply love it. The show needed a more action-packed OP and this, in fact, draws me into the show more rather than the more dreamy and mystical one that was present before. But, what the heck, there are tons and tons of spoilers in the OP, though, I admit, this makes it better and not worse.

Archer and Shirou are fighting..but..in Unlimited Blade Works mode?
A premonition of what's to come? But why are they fighting?
(the gears look cool though, I love how UBW looks)

Who is he? Another Servant? Possibly, since he seems to be fighting Saber..
Could it be Lancer (whom the show has completely forgotten about,
or so it seems..)

I still don't understand this whole concept of Saber being King Arthur.
a. She's the wrong gender (though sometimes she's referred to as Arthuria..strange..)
b. um…she's the wrong gender!
es un gran anime con una gran historia felisito a los creadores y pido consejos de como hacerlo por que yo estoy creando una historia
The Shirou/Archer eyecatch in the new OP is a nod towards the Shirou/Archer fight in the game, I suppose.
And yes, Archer be Shirou — although there’s only circumstancial in the anime. Aside from what you mentioned, his calling Ilyasviel “Ilya” (only Shirou does that — Rin calls her Ilyasviel) and his ending music (called “Emiya”) come to mind. Although it’s a bit hazy for me, I seem to recall he used the power of the Grail (by becoming a Servant) for the explicit purpose of finding and killing off himself. Or something like that. Think of it as a great big sulk.
I haven’t played the game yet, still waiting for the translation to be finished. Noticed Ilya’s shock at Archer calling her ‘Ilya’, it spurred thoughts of the Archer=Shirou theory. Never knew about the ending music and the fact that he uses the Grail.
Archer is actually an adult Shirō Emiya, the main character of Fate/stay night. He is ultimately the winner of the 5th Holy Grail War, and is a living incarnation of the “Hero of Justice”. The Throne of Heroes, which preserves the souls of all Epic Spirits, exists outside of any conventional timeline, so Archer can be called from any point in history. The amulet Rin used as a catalyst in Archer’s summoning was the same jewel she used to save Shirō’s life.
His history differs from the Shirō in any particular storyline, although he is most similar to the version who progressed through the Fate scenario. Archer remains the former Master of Saber, but there is a strong possibility that Avalon, one of Saber’s Noble Phantasms, was never removed from his body. He cares deeply for Illyasviel von Einzbern, and his inability to prevent her death one year after the war’s end is his greatest regret. At one point in his future life, Archer finds himself incapable of saving the lives of several hundred people; to change the course of their destinies, he forges a pact with the world, whereby in exchange for a miracle he is obligated to posthumously and indefinitely serve humanity as an Epic Spirit. Later, one of those spared hundred uses him as a political scapegoat, executing Archer under the accusations of war crimes for wanton violence and destruction.
Throughout his heroic career, Archer struggles with the same doubt his father, Kiritsugu, acknowledged many years before; when the life of one person is saved, another person must be left to die. After fighting in many future battles, he is horrified to discover himself needlessly slaughtering countless numbers of the nefarious and innocent alike. In time, the once idealistic Shirō comes to the pessimistic conclusion that attempting to preserve all human life is futile, for the few must be sacrificed for the survival of the many. Accepting this concept frustrates Archer, for it betrays his life’s conviction and renders his selfless dedication to defending mankind worthless.
Broken, bitter, and angry, Archer gracefully accepts his death, gambling his afterlife on the small hope of being summoned as a Servant back in the 5th Holy Grail War. His rationale is to kill his younger self and cause a time paradox, which Archer hopes will spare Shirō the anguish of realizing his ideals are a sham, even at a cost of ripping the fabric of reality asunder.
Just to anwser you, i played the fate game. The father of saber wanted a boy, and he got a girl. Insulted, he asked merlin to do a spell on arhuria. Making her to be seen as a man. Since women cant be king, this way his only way. And so that’s why she is a girl and not a boy.
But the real anwser, saber was a boy when the maid fate. But after saber turned to be a girl. Why? Sex scene for sure, that’s the outside reason.