Act 3 will end in the next chapters, Onigashima almost on the side of Mount Fuji and the Skull is also more damaged F for the SN
My theory is about to come true!
Mount Fouji is actually the neck and spinal cord of that skull Giant human.... Kaido and Big Mom will revive that Giant, and Oda for the first time EVER reveals one of the Ancient Weapons (Pluton or Uranos) and we have an ancient weapon in the war...
Oda is slowly making Kaido's and Big Mom ambition during the announcement come to fruition... They announced that they will be in alliance (happened, and it seems going well so far)
they announced that Kaido will take Onigashima to Wano... and make it the capital, people interpret it in different way because they COULD NOT BELIEVE that Kaido will lift the entire island and drop it there as new capital, so they just assumed it will become the capital without moving the island... (BUT Kaido lifted the island and will drop it)
They announced they are going after Ancient Weapons (Probably will succeed in reviving one of them, it's not soon for the story to involve them)
They will terrify the world (will probably happen)
Lastly, plan on going to Laugh Tale and becoming pirate king.... (Will fail by Luffy's and co)
So, Oda will build EVERYTHING, and then at last goal, Oda will take everything away from them.... So I theorize this skull giant will be revived and will be an ancient weapon... we will see the huge sword in action.... It's gonna be davastating... requiring Marines to be part of this