So far, the characters have been grouped based on a theme.
Wano characters
WCI characters
(Both are even lined up with Kaido and Linlin respectively)
Dressrosa Allies (Law, Barto, Corazon)
Warlords (At least partly)
This could help us determine how Oda places the others. I imagine all the Marines get boxed together, Straw Hats are a given, and Shanks and Teach occupy similar spots as Linlin and Kaido, looming at the corners of the mural.