OKIE DOKIE, i think i figured out how xion comes back.  I figured it out while thinking about the Rikuplica, but hes not important right now.

 

SO heres how it happens.  In the last trailer, we see Xemnas and Ansem SOD giving Sora the opportunity to open up his heart and get Roxas back.  And this is when Xion comes out.  In 358/2 days, Xion says she is in Sora.  Well so is Roxas when he returns to him.  So once Sora gives himself up to get Roxas out, Xion will also come out.  Whether or not ppl remember her....ask Namine.  YEP YEP I think I figured it out.  Time to listen to Musique Pour La Tristesse De Xion again (maybe another remix!!).

 

And she is coming back because Namine said so in Coded.

With Repliku, some Youtuber said there's the symbolism, which tells me he isn't there, thus Riku's internal struggle. But he could end up being there, physically or mentally-maybe metaphysically. Roxas makes sense, but Xion is a separate matter I think. Oh wait, never mind. Just remembered the ending of Days. She's basically inside of him in memories and is the only one who held out the longest, I think. Namine, however is that SPECIAL exception where she could remember anyone, regardless of the chain of memories mechanic. Sora can can bring her back, because he's the SPECIAL exception where he said to Namine that regardless of if he had false memories, the memories will also be there. In this case, Roxas transfers the memories in DDD and thus goes into Sora's heart, extracts it somehow, and poof, screw the chain of memories mechanic, because as long someone has the promise or memories in there heart, well, Sora saying it all does it. Enough said. My original thing before I said all of that was, Xion was made in Castle Oblivion, thus Sora would have to go back there or something, because that's where she was from and the whole data thing with Roxas was about only about him and not Xion. But I remembered about the end of Days and yeah, it COULD happen. Another point, how is everyone going to come up with a plan to get Namine out of Kairi? Like, she's the other person that has the right to be their own person. Also a key plot character. If anything, with her powers, the power of memory manipulation, she basically can read into peoples hearts and see what happened to peoples pasts. Meaning, we can figure out why Ventus is young, and why Xehanort is doing all the evil he's doing. Actually, she can be great use of finding the black box! *GASPS* OH my GOD! But then again...depends when Namine is going to be at thing. Because someone could of ended finding that thing anyways before she's released. So sorry for all of this. You got me thinking. Still thanks anyway.~

Xion is based on Sora's memories, she doesn't exist in any capacity right now. Presumably Namine will have to do something to draw Xion out

With Repliku, some Youtuber said there's the symbolism, which tells me he isn't there, thus Riku's internal struggle. But he could end up being there, physically or mentally-maybe metaphysically. Roxas makes sense, but Xion is a separate matter I think. Oh wait, never mind. Just remembered the ending of Days. She's basically inside of him in memories and is the only one who held out the longest, I think. Namine, however is that SPECIAL exception where she could remember anyone, regardless of the chain of memories mechanic. Sora can can bring her back, because he's the SPECIAL exception where he said to Namine that regardless of if he had false memories, the memories will also be there. In this case, Roxas transfers the memories in DDD and thus goes into Sora's heart, extracts it somehow, and poof, screw the chain of memories mechanic, because as long someone has the promise or memories in there heart, well, Sora saying it all does it. Enough said. My original thing before I said all of that was, Xion was made in Castle Oblivion, thus Sora would have to go back there or something, because that's where she was from and the whole data thing with Roxas was about only about him and not Xion. But I remembered about the end of Days and yeah, it COULD happen. Another point, how is everyone going to come up with a plan to get Namine out of Kairi? Like, she's the other person that has the right to be their own person. Also a key plot character. If anything, with her powers, the power of memory manipulation, she basically can read into peoples hearts and see what happened to peoples pasts. Meaning, we can figure out why Ventus is young, and why Xehanort is doing all the evil he's doing. Actually, she can be great use of finding the black box! *GASPS* OH my GOD! But then again...depends when Namine is going to be at thing. Because someone could of ended finding that thing anyways before she's released. So sorry for all of this. You got me thinking. Still thanks anyway.~

Maybe repliku could be present the same way the joker was in arkham knight as a ghost who constantly gives advice but cant physically help