Fair enough. As long as it doesn't actually happen I guess it's not really an issue....
Not in any of the Fate universes, but it does come up in others.
Generally no, but I've had to deal with hear trouble and did research on it because of that. The transfered heart (due to the size difference between the two of them) would press more against her left lung than the original did. While this would cause discomfort, it would not be lethal and the body would adapt. Heavy exertion wouldn't be a good idea until she had, and were it a normal heart, too much cardio even afterwards might risk bruising the heart (not fatal but painful and risky) but... well, it's not a normal heart. However, the size difference isn't that big in the first place. It's large enough to have an effect, but not to be ridiculous.
Ah, OK.
Well, a bit of discomfort is nothing compared with what Zouken is putting her through anyway....
Pretty much my thoughts.
Hrm. Then I have to address that... any thoughts?
Well, I guess it depends. Shirou survives with Archer's arm attached to him, after all. And, the fact that he's even more of a normal human than Archer might help. He might not have quite the same spiritual presence.
That was my original thought, but there is the problem that this guy is inherently tainted. He was tainted deliberately in order to increase his resistance to further tainting (specifically Dragon Age, he gets drawn into the Gray Wardens) which actually makes him immune to the Blackening that the Shadow performs (though it still kicks him out of existence), but this brings up the problem of how Sakura's body would handle that. Even if there's no real 'spiritual' component to worry about or even any of his blood still in the heart, this is magic we're talking about here. That and yay vampire.
I haven't gotten through Heavens Feel route yet personally so I'm not sure how that would affect her.
The problem is that the way you're going about it is going to look horribly contrived. There is no reason Rin should die over a completely powerless servant when the servant is the logical first target. Especially since Rin is probably a match for most masters and can hold off a servant for a bit. It happening once makes sense, it happening multiple times seems just bizarre.
The first time is purely him being 'Lucky' and her not, Gilgamesh rains death down on a large group (most of the other Servants and masters) at once, and while Archer OC and Rin both go off to the side as 'Masters' to stay out of the fight, Gilgamesh doesn't accept this since Archer OC is the one who essentially started the fight. Rin just happens to get hit before Archer OC and he has essentially no prana or Independent Action at that point.
The second time, when Berserker attacks, Rin actually tries to protect Archer OC since he has
no combat ability whatsoever and Saber isn't able to hold off Berserker on her own. At this point, while Archer OC isn't a coward, he's not Shirou either and panics, getting and staying out of the way, and Rin gets bissected. Rin reasons that since Archer has no combat ability and she's the one who summoned him, he's her responsibility (he's explained his situation to her).
The third time, he's barely competent and equipped enough to fight and almost gets killed when Berserker nearly bisects him. The only reason he
isn't bisected and very very dead is because Berserker had to hit through a shield and his armor and Rin uses a Command Seal to make him escape, but it kicks him out of existence and back into the kalediscope, removing him from that world. That and in the next world his companions use up all of their healing magic to save him.
The fourth time they're attacked by Berserker and Illya early on as per canon, but when Shirou leaps in front of the attack meant for Saber (since even between the two of them, they can't handle Berserker for obvious reasons), but for reasons that Archer OC doesn't figure out, Illya doesn't have Berserker stop after basically bisecting Shirou. Archer OC has Rin make a contract with Saber on the spot, and Saber ends up using Excalibur, Archer OC stops draining Rin's prana to avoid hurting her... but finds himself fading almost immediately since he has so little prana... but decides to go with it since he's not a viable Servant and it's not killing him.
(This one was revised from the original based on the whole Independant Action thing)
The fifth time, after a solid showing, he gets hit by Rule Breaker and avoids letting Caster take him as a Servant, using his anti-magic blade to take her down before he dissapates. Rin being unable to make a new contract with him before he fades out.
(Again somewhat revised from the original, this time he has the time to actually take out Caster rather than just immediately fading out).
The sixth time, after a decent showing (and getting Rider/Sakura involved as an alliance of Rin, Shirou and Sakura) he gets opened up by Berserker again and Rin tries to force him to turn into Astral form using a Command Seal to save him. since he hadn't mentioned what he was to her this time (a mistake), and ends up getting forced out of existence again instead.
The seventh time, after a very solid showing it ends up with a fight of Archer OC and Berserker vs Gilgamesh. Gilgamesh pulls out Ea and between Berserker and Archer OC's armor/shield/magic of ridiculousness they survive a mid-power blast from it, the surprise of Gilgamesh giving Archer OC enough of an opening to shoot Gilgamesh in the head with a technomagic gun... and then realize that Rin and Shirou and Sakura were all behind him and Berserker when Gilgamesh used Ea... Archer's newly E-rank Luck making a showing. He survived without his master long enough for the combat and uses up the healing potions he has to try and save the others, starting with Rin and then moving to Shirou and then to Sakura, but he fades out before he can since his link with Rin was broken.
(Again, slightly revised)
The eighth time, between himself and the others, they take out Berserker, Caster, Assassin, and Lancer, but before Gilgamesh shows up, the Shadow makes an appearance and drains Archer OC of prana and tries to blacken him... but ends up just making him dissipate. Archer OC never realizes that Sakura was the source of the Shadow.
The ninth time is much like the Eighth, but Saber is blackened first and Archer OC ends up having to hold her off while Shirou and Rin try to save Sakura. But Saber is basically the worst possible opponent for Archer OC and while he wins, he ends up using the last of his Prana to do so and Rin was killed by Psycho-Sakura so Archer OC fades out.
The tenth time, Archer OC ends up essentially winning the war, this being the time that he actually saves Sakura since he knows to do so, the other Servants being non-issues at this point.
But as for the servant thing... why? If that was the case, and Saber was just a soul summoned by the Grail like the others, why couldn't she assume astral form?
Well, Saber is still counted as "alive" but, at the same time, she is still clearly a spirit. She requires prana to survive and she just fades into nothingness when she is defeated. I also find it unlikely that killing Saber in the Grail War would prevent her returning to her body.
True.
Sorry
Though he picks up so many abilities because he does need them, but they're far more often tailored to the realm which he's in at the time. The ability to alter Fate and Destiny and being immune to it isn't worth much in F/SN, but in Kingdoms of Amalur it's the defining trait of the protagonist. A lot of the abilities are like that.
Ah, OK.
Well, one issue I can see here is that a massive cross-over fic like this with an OC protagonist is going to require knowledge of a lot of different works to be readable.
Oh yeah, I've been hashing out how best to make it easier on readers who are new to works to ease them into it. I've got a couple ideas, most of them involve people that he expects to look one way looking somewhat different so he does have to describe them in his head, as well as the OC himself being clueless as to where he is and learning with the readers.
It's no problem, I'm happy to help, and you did come over to my forum to discuss it, so helping you out is the least I can do.
Still, I appreciate it ^_^