So there is a scene in KH3D and the title is "The Box's Contents" and Xemnas is talking to Sora in this scene and says some stuff that made me think and I think it foreshadows the box in Back Cover/KH3.
Wait, maybe he does - Xemnas/Terranort. (bonds and stuff confusing) but doesn't just Terra know if that is the case? Even though Xemnas realizes and sees Ven in Roxas.
I understand where you are coming from. It is interesting since you mentioned it. As of now, I would just take it at face value and see it as nothing more than Xemnas making Sora check to see if what he feels and remembers belongs to him and not Ventus.