OMG I just realized where Pluto is in the mural
We can clearly see Poseidon commanding the Sea Kings as represented by Shirahoshi at the left. We can clearly see Uranus shooting vertical beams similar to the ones we saw at Lulucia at the right. One might wonder, where is Pluto?
> Is it the big ship Zunesha is carrying? Nah, we know it's the Noah Ark.
> Is it one of the dark creatures? Nah, we know it's a battleship able to destroy lands. Then, where is he?
It is Franky!! He is looking at the mural, not knowing he is part of the mural too, as the future wielder of the Battleship Pluto and the only person on Earth who knows about its plans and functionnings. If it's true, it's such a nice detail added by Oda to foreshadow Franky's role in the coming war, especially as he is darkened just like the three other monsters who look like they're facing him.