I'm really curious about how Ephemera and Skuld are going to be included in KHIII, yeah. I really hope it's in a way that doesn't somehow diminish what they went through in Key (there's a small part of me that almost wishes that they'd just be some of the first to rebuild the world, and died naturally after that; but we know they're going to be put into stasis, or something, and be a part of III) and without making death feel even more pointless in this series than it already is, but we'll see.
Hopefully Ephemera and Skuld don't die at the end of X and are just put somewhere safe, ala Ven, so it'll feel less cheap when they come back.
Also, you were saying above that there's no way to compare Player to Kairi, but there is: the fact that they both never have any lines, really. Unfortunately. (Note that I'm totally joking here, but sadly it's true; at least where Kairi's concerned. Player is/was supposed to be silent, of course... Kairi? Not so much.)