If you look at the cave drawing of the Gerudo on horseback, you will notice that the "spear" is actually a trident, which further suggests that this drawing is the depiction of Ganondorf as he has been associated with the trident before, especially when in the form of Ganon.

Also to further suggest that the "mummy" is the Ganondorf from Twilight Princess, recall that in Twilight Princess, the Sword of the Six Sages, as well as the Master Sword, both pierced Ganondorf at the spot that the arm/hand is now touching in the BOTW sequel trailer above. My guess is either this is the arm of the Twili (which Midna is a part), or perhaps even associated with Zant, who is attempting to resurrect Ganondorf. Either way, I am convinced this sequel is associated with Twilight Princess.