I'd figure that Marluxia and Larxene would recomplete back in Daybreak Town/modern Scala Ad Caelum, but surely anything's possible there, since we don't know how/when they became Nobodies. Demyx doesn't feel as enigmatic to me as the others, though, because he doesn't "die" again in KH3. He just warps off to go chill somewhere, and he has his own Gummiphone alongside the other allies to Sora & co., so I don't see why Riku, for example, can't give a shout out to Demyx and he could pop up when and wherever, y'know?
It's like when a work friend finishes their shift... You may not know EXACTLY where they're going next, but you know that metaphysically, they're right around the corner and not missing.