Lea actualy has a chance of returning though simply because it was confirmed he became whole after KH2.
Axel/Lea is a character that doesn't know how to stay dead because the power of fan-service keeps him alive. So in that regard I knew he was coming back. Officially though it was never stated that he would. Plus you just know that
convenient Directors Report about nobodies possibly returning to their real selves if certain conditions are met was made just so Nomura can keep whoring Axel out. Or rather Lea now.
Xion should atleast be mentioned or Sora should atleast learn who she was. Roxas and Namine can obviously come back, they still exist inside of Sora and Kairi. Honestly it's not bad if Roxas, Namine, and Lea return if they are there to help Sora with his next journey.
Remembering is one thing, but actually deus ex machinaing a way to pop out of Sora all of a sudden to be her own person when she is nothing but a bag of
memories that are in their rightful place is something else entirely. Namine and Roxas can not obviously come back as you put it. That was the whole point of them merging for good after the Xenmas's( fake) last battle. They are
one with their human-selves. They
are Kairi and Sora now. Smiling at each other on Destiny Islands
through them only meant, just like they themselves said, that they will see
each other as how they looked as nobodies. Not that it was hinting at some retcon way for them to walk around now since they are completely whole. Also I could have sworn data Namine herself says that her real self is gone.
I mean not only will Nomura have to come up with a big asspull to even
begin to explain why Namine, Roxas, and Xion( who all went peacefully back where they
belonged in the first place) are now out out of their host, it will retcon the past three games like crazy. Is that the kind of stuff you'll tolerate just because it's Kingdom hearts?
Also Nomura himself said it would be difficult to bring her back. Implying that it was never his intent to do so in the first place,and thus if he brings her back anyway it will have to be vie some bs fan-service. Remembering(retcon but let's at least give her oh so tragic self that) is okay. Actually
physically bringing her back, and
completely negating what we had to go through with her in Days is not okay at all.