Zeltik made a video saying he believes the new figure in the stone depictions is Hylia, but I agree that it's possibly who the green arm belonged to, and not Hylia. If it's Hylia, then how did Hylia make an agreement with herself, i.e. Zelda in the stone depiction? Zeltik's theory isn't well supported in my opinion, even with the idea in Skyward Sword that Hylia is an interdimensional being, which I don't think she is really.
On another note.
It looks like the new malice dissolves metal, which is why it dissolved much of the master sword, and in the stone depictions it has killed soldiers and destroyed their swords and shields (if you look at the depictions of the soldiers in the trailer, the soldiers outside of the new malice have shields and swords, but the soldiers inside the new malice have lost their sword or both sword and shield). Maybe this is why all that is left of the Zonai are stone structures. The game developers could explain that this is also why all the ancient sheikah technology, i.e. divine beasts, guardians, weapons, shrines, towers, sheikah slate, etc... are immune from this new malice since they're not made of metal. That's why 10,000 years ago they were able to seal away Ganon because the divine beasts and guardians were immune from it, but 10,000 years later, Ganon/Ganondorf changed his malice to take over the sheikah technology in BOTW.