Its a common theme that Oda spreads everywhere.
Elbafs three realms.
Land, sea and air...
3 ancient weapons...
Zoro has always been hell themed.
Luffy is sun themed.
Sanjino has to fill whatever is left.
This feels retrofitted. Zoro had some demonic imagery long before Wano through Oni Giri, cursed swords and Asura, but “King of Hell” only became an actual identity once Enma entered the story. Among other themes like buddhism or monkism? Not sure, not a Zoro scholar.
Ashura alone kinda does not prove Oda had some fixed Underworld role planned for him, especially when Sanji has Diable Jambe, Hell Memories and Ifrit Jambe, while Robin is literally called the Devil Child.
Luffy had dawn and liberation imagery, but the literal Sun God angle was introduced very late and then pushed heavily from Wano onward.
Sanji is much harder to place in one category. He has links to the sky through Sora and Sky Walk, to the sea through the All Blue, to royalty through Germa, to science through his body, to fire through Jambe, and to food through being a cook.
Oda has given him too many overlapping motifs for “Heaven” to feel like an obvious destination.
Love is probably the closest thing he has to one central theme. Sora preserved his emotions, his kindness separates him from Germa, his fire is tied to passion, he is woman crazy and he literally says love is stronger than light.
I wouldn’t mind something like „Devil of Love“