So as we all know, Repliku is dead. No doubt about it. The problem here is, according to our director, the is no such thing as death in this universe.
As he was "dying" he asked where he would go, and I'm wondering that as well. I don't think he would just fade into nothingness, he did have a heart after all. Anyway onto my theory...
When Riku said "Maybe your heart will go to the same place as mine" it gave me the idea that maybe, because of the Repliku's connection to the original, his heart took refuge within Riku himself as his replicated body faded into nothingness. This wouldn't be too unusual, as Eraqus' heart went inside Terra, I don't think it's too impossible for Repliku's to have gone into Riku.
It bothered me that it seemed like almost everyone in the series has been foreshadowed to return EXCEPT Repliku, but maybe him coming back isn't totally impossible. The director said that everyone inside Sora are only sleeping, waiting to be woken up when they can return the world. So maybe Replica is sleeping too, inside of Riku, waiting to wake up.
I've actually brought this point up in the past. I agree with you.
If Repliku is like Xion, then his fate should be comparable to hers. She became a part of her original, so naturally he should become a part of his original.
But people frequently bring up a counterpoint- that Repliku is
not the same. He is made of "data" while she is made of memories. To which I say, hogwash.
First and foremost, there is no such thing as a generic "data." Data is comprised of
something. Whether it's the pH level of an acid or gigs on a computer, you need to have some kind of units by which that something is measured. And in the case of Repliku, that something is memories. Vexen sampled Riku's memories during that battle, much in the same way that Xemnas sampled Sora's memories when they fought in Hollow Bastion.
In fact, the novel confirms this, as we read Vexen's thoughts when he fights Riku (courtesy of gp♥):
If we did things according to my research, it is theoretically possible to transfer memories and abilities to a doll without the power of the witch.
[...]
With every swing of his blade, his power and memories are being accumulated as data.
[...]
Now all I have to do is plant these memories into that doll, for a start…
But it should be obvious, even without Vexen's thoughts, that Repliku was made from memories as well. Because they're both
replicas, it makes no sense that they could somehow be replicated by completely different means and still be classified as the same being.
So then this begs the question of whether or not Repliku had the same effect on Riku as Xion did on Sora. And the answer is a resounding "maybe."
We have potential evidence that Riku lost some of his memories over the course of traversing Castle Oblivion.
Zexion states that the memories of his friends are gone, with only dark memories remaining. Granted, Riku remembers their names, but he does not object to this. So some of his memories may have been leaked to the Replica.
And then there's the possibility that this was an outright bluff. Unlike Riku, Sora's memories were being manually pulled out by Namine, which allowed them to leak into Xion. Plus, Sora already had a Nobody in addition to a replica to which his memories could leak. So Riku is a different case.
Either way, though, there's no doubt in my mind that Repliku was at least made of some of Riku's memories, replicated or otherwise (and Namine contrived new memories on top of these). Therefore, I do think he became a part of Riku.
But he will still be forgotten about by the writers.
That said, I don't think there's any point to bringing the Riku Replica back. First of all it would ruin the point of his character development (coming to terms with his existence) and second of all, why? He has no friends, doesn't he?
Just as there is no point in bringing Repliku back, there's no point in bringing Roxas, Namine, or Xion back, as it would ruin their character development as well. That's why I think, or at least hope, that freeing them from their torment doesn't mean bringing them back.
But if that's the case, then there's no reason why Repliku shouldn't be freed from torment either.