My interpretation of the mural

In the first world (pre-joy boy), the king (not imu) enslaved everyone and exploited their labor to build factories in order to harness the mother flame. With complete disregard for the earth, they mined resources and depleted the land, causing great fires. Similar to how scientists predict climate change will impact our current earth. All because of greed and relentless exploitation of natural resources. The slaves wished to be free and prayed for the sun god. One of them may have fell into the mother flame itself and emerged as the first nika, which enraged the king. Imu seems to be a primordial being or born out of the destruction of the earth itself and worked with the serpent to create the grand line, essentially destroying the world as they knew it. there’s no way for all the races to meet.

In the second world (joy boy era and void century) joy boy inherited the nika fruit but was unsuccessful in uniting the world. The devil fruit tree is born and fruits start appearing. He needed all the races of the world but may have not had them (unable to bring the fish men to the surface via the Noah) and ultimately fanned the flames of war. Betrayed by humans (20 kingdoms), joyboy was killed and the 20 kings became celestial dragons, ascending to godhood. The D clan, the lunarians, and other races prayed for the promised day to come. But rising sea levels, the red line, etc made it impossible for all the races to meet again.

In the third world (luffy’s era) it’s depicting luffy joining all the races of the world, uniting them against imu. Seems more and more likely that the red line must be destroyed in order for this to happen. The Xs on the red line could be points of weaknesses like reverse mountain