That would only be possible with using Time Travel as Xion right now is only a heart (which on top is also forgotten by everyone) that sleeps inside Sora. Her puppet body was destroyed at the end of 358/2 Days so there's no vessel available and that's what Xehanort needs and wants.
Furthermore, as far as we know Xion never had a Xehanort seed, so Xehanort would not be able to control her.
He is the true and most future Master Xehanort, that has been confirmed both in-game in DDD, the DDD Ultimania as well as the Memorial Ultimania.
Xehanort is only concerned about the bodies which he can control and have as his vessels, so when he infers that Terra still "belongs to him" it can mean the body alone, as just Terra's heart alone cannot really be one of the GoL unless it gets somehow a new body to inhabit.
Vanitas is indeed a highly possible candidate, not only has he the golden Xehanort eyes, he also has a heart of pure darkness as well and was a servant for Xehanort for four entire years.
Vanitas' current status is unknown, all Nomura confirmed was that currently he doesn't have a physical form. Where exactly his heart is is also a mystery. During the final battle in BBS, Ventus engaged in the entire final battle because he did not want to merge with Vanitas, he fought not only to destroy the X-blade but Vanitas himself as well as he even plainly states before the fight. What happened to Vanitas' heart and consciousness after he was defeated has thus not been cleared up so it could be anywhere.
Ansem the Wise in DDD also speaks of "three young people", meaning Xion, Roxas and Ventus. I don't recall him ever mentioning four.
Dilan and Even are said to be still "unstable" by Aeleus, so they might still be possible candidates. Dilan even more so since Xaldin had the pointed ears.
I agree with the notion that Aeleus and Ienzo are most likely headed for a different role though, possibly even as new sidekicks for Lea until Xion and Roxas are restored.
Despite my fondness for the story of KH as a whole and my like for most of the characters, if there is one complaint besides the sloppy writing I can get 100% behind it is the atrocious handling of the presentation of important information regarding the mysteries of the story.
Kojima's Metal Gear franchise also has a very complex and interwoven story but since I didn't play the series I can't say if Kojima handled it any better than Nomura does with KH.
Nomura and Hideo Kojima are good friends in real life and Nomura also got advice from Sakaguchi (who Nomura reveres as a sort of mentor figure) to make KH more complex in order be able "to compete", so these may actually explain why the KH story is made this way.
It is true that the heart is the most important part of a person (it's also the only part that is essentially immortal due to memories being immortal, it can cease to function if damaged too heavily, but never completely erased.), but in order to truly "return" to the physical "living" world a heart needs both a container (body) and a source of lifeforce/fuel (soul) to operate that container. Furthermore, to Xehanort,the body is right now the more important part for his purposes and that's why his statements go towards the bodies.
Just before Sora falls into a coma Young Xehanort says that his heart will sleep inside the Darkness forever, while his body becomes a new vessel for the true Org, showing that the Xehanorts dismiss the hearts and only focuse on the bodies which they need.
This "Xemnas" would be the Xehanort-seed that controls the body, yes, and it would allow Master Xehanort to return to his own body without actually losing the body (Terra's) he was working so hard to acquire in BBS.
The whereabouts of Terra's own heart is then where it gets tricky as it being inside Riku, still inside his own body (the recompletion process that put Xehanort's heart back to its true body should have done the same for Terra after Ansem and Xemnas were defeated), somewhere in the Realm of Darkness or even somehow making its way to the Lingering Will are all viable possibilities.
Then one has to keep in mind that Eraqus' heart is in the mess as well, opening more possible cans of worms.
Yes, Roxas, Xion and Naminé do not have their own bodies because they are new existences derived/spawned from other already existing ones.
It's one of the main problems that comes with these three as for them to "return" as true own individuals there would be the need to somehow prepare/create true functioning bodies for their hearts to inhabit.
We do not know how this will play out though, the only information we have regarding this since Birth by Sleep is that Ansem the Wise's data is somehow the key to achieve it and that Sora is apparently the only one who can do it.
One possible solution might be actual true cloning, for example creating two clones from Kairi's DNA to provide bodies for Xion and Naminé while creating one from Sora's to provide a vessel for Roxas. Another could be that their hearts get awakened and released, then Kairi gives them a MPoHh (magical Princess of Heart-hug) to give them a true human form like she did for Sora in KH 1.
In Terra's case however, there already exists a body that his heart belongs to, just like Aqua and Ven have too.
If it would be easier to plant Terra's heart into a new body somehow and leave his old to "Xemnas" the Xehanort-seed is right now an unanswerable question as we do not know how it will work, but it would certainly be a possibility to keep in mind which could also serve to have Terra fight on the right side alongside his friends and allies and not forcefully against them.
Heck, it is even possible that his heart might be planted into the Lingering Will temporarily until they can truly win back his own body.
Knowing Nomura, this might even lead to Terra somehow addressing the "Terra"-part of "Xemnas" and with Sora's help trigger something in "Xemnas" to truly wanting to become his own person, leading to some of Xehanort's own seeds rebelling against him as they can eventually develop into their own individuals, again displaying Xehanort's main weakness of underestimating the true power of the heart.
As far as I can see yes, as Terra's role does have big ties to the 13 Seekers of Darkness either way.
In terms of who the six unknown ones are there can not be done much else than to list possible candidates and exchange pro and contra arguments as to how and why they could be one of the thirteen anyways.
What we have done here concerning Terra could be done in the same vein for Vanitas, Riku Replica, time-travelled evil KH 1 Riku and any other possible seeker candidate.
