I think is "safe" to assume that the special power of Xehanort's Keyblade is the power to "split hearts", we all have seen he do it with the organization in a Horcrux scenario, and with Ventus/Vanitas , but we thought that this could be one hability inherent to all keys, so technically even "Mickey" could do it if he was evil, but that doesn't seem to be the case anymore.
Xehanort plan seems to be collect those special keyblades that have habilities that could help his plan:
1) Xehanort keyblade: Can split one's heart, making he able to be imortal by changing bodies (not only for vanity sake, but he should not know how much time he would need to access kingdom hearts).
2)The X-blade: Has the power to summon Kingdom hearts giving him the power of a god, I think that due to his experiment with Ventus/Vanitas both Sora's and Mickey's kingdom key -keyblades are result of the destruction of the X-blade, thus why only both of them together had the power to close Kingdom hearts (if it was only a matter of two keyblades, King Mickey already had his, even if he needed one from the dark realm and another from the light , he could trust Yen sid to use his one at the light side, so probably IT NEEDED TO BE BOTH KINGDOM KEYS).
3) The key of people's heart: Not only it has the power of opening the door to darkness where lies Kingdom hearts, It also has the ability to "open one's heart to the darkness", a good power to have when you can put yourself inside the other person's darkness and take their bodies when they are consumed by darkness, work perfectly with his first Keyblade, and probably why he created the heartless in the first place (Is the same thing done artificially).
Other important Keyblade is Eraqus Keyblade that could seal Castle Oblivion/land of derpature, inherited by Aqua and now Sora, probably has some ties with the Keyblade war as well.

Soul Eater isn't a keyblade

Time flows differently in each world

It is true that Soul eater is not a Keyblade, but it was used as medium for Riku's way to dawn, just like the wooden sword was used to make Sora's Kingdom Key.
But the fact that soul eater is NOT a Keyblade and has zero conection with Zehanort just shows how much it's eye is not connected to Xehanort's Keyblade as well.

That's not confirmed. We just don't know where Soul Eater REALLY came from, so that's not necessarily true.

we know it's not a keyblade, they call it a sword, and Riku is able to call it even after getting Way to Dawn
or look at it this way, when Riku gets possessed he gets the keyblade to people's hearts, this shocks Maleficent, and Riku/Ansem says "That's right, a keyblade"
Why would he make the distinction, and why would Maleficent be shocked if the Soul Eater already WAS a keyblade? The whole point of Riku taking Sora's keyblade back was that he DIDN'T have a keyblade

You misunderstand. I am indicating that we never see Maleficent show any in-depth knowledge of Soul Eater's origins, so it's not necessarily *unrelated* to Keyblades, that remains to be seen

... but there's absolutely no indication that it has any relation, but plenty that there isn't, so why assume that?

That's incorrect. There would be "absolutely no indication" if Soul Eater had never become Way to the Dawn, and did not sport the eye that multiple dark-themed keyblades share. It is still a valid theory.

His ability to be a Keyblade wielder comes from Terra, but his medium is the Soul Eater, that was given by Maleficent...
Truth be told that Riku COULD have been tempered by Ansems Heartless (that meet with him in the islands), so that is why his darkness has an eye even before being a Keyblade.
But i really think that the eye theme is only for a "coolness" factor, Nomura draw it once and than made a theme for all dark keyblades, It can be retconned afterwards for being Xehanort's Keyblade, but will be a strech!
In favour of this theory is that Dark Kingdom Key (the only Dark Key not directly connected to Xehanort acolytes) is a Dark Realm keyblade that does not have an eye.

Source:interview

It didn't become it though, we don't know where Way to Dawn came from, but we DO know that Riku can still summon Soul Eater despite having Way to Dawn, he did it in DDD. The eye thing is associated with darkness, and that's the extent of it. Terra uses eye and hadn shaped darkness attacks.

You're not understanding the quote, when he says it's an intermediary, he's talking about how it was what Riku used as a weapon before switching it for Way to Dawn. Soul Eater is just a sword presumably given by Maleficent, but we don't know where Way to Dawn or Soul Eater officially came from. But they are separate entities

Well, I didn't play DDD, but as far as my understanding goes the "Keyblade", is separated by two "existences":
1) There is a need of a Wielder with strong heart, and the inherited ability to use one Keyblade
2) The keyblade once is forged exists even without the wielder, so It can be passed down to others, or being in the Keyblades cemetery, or locked at some world.
The thing is unless the keyblade is passed down directly to the new wielder (Like Mickey received one from Yen Sid), there is a need to "forge it", for this the keyblade needs a medium, for Sora it was the wooden sword, for Riku Soul Eater, for Roxas the "struggle bat":
As we can see here

There has never been anything that says that another weapon is required to make a keyblade. It simply makes for a more striking visual to have an innocuous weapon transform into an elaborate key
(though in the case of Roxas, it looks like the struggle bat itself was transformed into a digital keyblade, it's also worth noting he uses a stick to accomplish the same thing later on, so if anything the use of another weapon is simply to get the character in the correct mind set)
But here's the thing, there's no transformation going on. Riku still has Soul Eater, again, he summons it in DDD. So even if another weapon is required to summon a keyblade, this would suggest that said weapon could be anything, and is merely replaced by the keyblade, not lost

I distinctly remember reading somewhere that Maleficent made Soul Eater and gave it to Riku. I really don't feel like looking for the interview, and there's no point to, since I'm agreeing with you. It's strange that this conversation became about this subject in particular. Sora used a toy sword before he got the Kingdom Key, doesn't give the toy sword any significance at all, except for that great scene in Kingdom Hearts when Riku tosses it to Sora like he's nothing, that was great.

And spoiler I guess, but at the end of DDD, Axel gets a keyblade himself, yet nobody assumes his weapons became said keyblade. Just because it's Riku and has an eye thing on it, doesn't mean that Soul Eater has any more significance than Luxord's cards.