I'm hesitant to believe that Nanashi is going to be a vessel for Nanashi, mainly because Nanashi's thing is that he's a wanderer from another another land and the Goddess Clan seem to only reside in things within Britannia's borders.
From the relevant characters and or items that have left;
Veronica; Not a terribly important character, then again neither was Margaret, but her possession would round out the Princesses each being possessed by a member of the Goddess Clan.
Excalibur; Could be the Supreme Goddess or Mael, but given the amount of power we've been told the Demon King has, I'm starting to believe that the powers of the Supreme Goddess could be sealed in a single blade. That being said, it might be possible that, like the Demon King did for the 10 Commandments, she fragmented her powers and Excalibur was a piece of it. If that's not the case, then I'd say Mael resides in the blade. If that is the case, that might imply that there is more to Mael than we think as s/he deemed Arthur a worthy person to be his vessel, without using brainwashing.
Estarossa; This is a theory I just thought of not too long ago, but what if Estarossa is Mael and, for whatever reason, was brainwashed into thinking he was a demon or, more accurately, the son of the Demon King? I admit, it's a crackpot theory, but given Cusack's feelings that Estarossa is out of place and Elizabeth's seeming connection to him, which appear to be residual memories from the both of them.