At the moment the most obvious couple would be Shiki and Rebecca, since they are the ones that show the most interaction, I don't know maybe something between Weisz and Homura is possible, although with Weisz we have also seen some acts with Hermit and from what I see Witch He doesn't mind showing her body, which if Weisz was Homura and Rebecca seems like all the girls he relates to are not human.
With Weisz and Rebecca, in the case of the Old Man it is like a relationship between father and daughter, and in the case of the Young man it is like a relationship between Big Brother and Younger Sister.