The new chapter makes me think one of two theories is true:

Ope Ope fruit and Blackbeard theory:

>Rocks sword attacks are known for sending items flying around
>Rocks cut a massive mountain cleanly and the top flew off rather than a slow anime slide
>Rocks disappeared without a trace
>Rocks put two fingers up when talking to Loki
All Ope Ope abilities, the two fingers is a reference to Loki's shambles pose. Rocks' unique swordsmanship was Ope Ope. He transplanted his soul into his own son in God Valley, both his son and God Valley are 40. Blackbeard being called "They" was Luffy and Zoro recognizing him with Voice/Breath of all things, him not sleeping is Xebec taking over in his sleep, the two fruits is because two souls.

Man Marked by Flames is Rocks + Davey Jones fruit theory:
>Kaido, Big Mom, and Whitebeard all dying burned to death is karma for them abandoning Blackbeard on the island to burn from a buster call
>Man Marked by Flames is so strong he can hold a stone alone against the entire world government because he's Rocks, one of the strongest men in history
>the ship was burned black on God Valley's buster call, same place Rocks got his burns
>the mentions of criminal underworld allures to that, MMBF is an underworld trader
>MMBF sinking ships is using the Davey Jones fruit to drag ships down to Davey Jones locker

In both cases, Rocks will fulfill his promise he made to Imu to return. Either through Blackbeard in his body, or as Man Marked by Flames and he never died.