[Theory] Another King. Spoilers from chapter 1187
At the end of chapter 1187, we finally learned what has been holding Sanji back from unlocking his conqueror's haki. Gaban told him "[he] is burdened with uncertainty and has to become a king in order to harness it".
When Zoro got his CoC, it came with Enma "which can cut through to the bottom of hell", and using the sword infused with CoC, he called himself "King of Hell".
While it does seem a bit corny to me, I don't think it's out of the question that Sanji will name himself "King of Heaven" when he finally unlocks his CoC.
In the Norse mythology that includes Nidhoggr and Ratatoskr, there is an eagle who sits at the top of Yggdrasil. The squirrel, Ratatoskr, carries messages to and from the Nidhoggr and the eagle. We have seen two devil fruits representing those mythical creatures this arc.
Elbaph has three realms. The underworld where Loki had been chained up, the sun realm where most people live, and the heaven realm which we know basically nothing about.
It would make sense if there were a third fruit representing the eagle at the top of the tree, in the heaven realm.
If Sanji is to become the King of Heaven, I expect he gets something heaven related, the same way Zoro received Enma in tandem with his CoC upgrade.
I've never felt that Sanji or Zoro should/would get devil fruits, but it certainly isn't out of the question for Sanji to get one that allows him to fly, since that's already his unique schtick within the Strawhats.
It could be a different "upgrade", but if Sanji calls himself the King of Heaven, I fully expect there to be something waiting for him in the heaven realm.
Thanks for reading. I await your verbal abuse, One Piece fans on reddit.