As BM states here, there is a connection between VOAT and the 3 eyed people.
Therefore could it not be the case that Luffy, Roger, Garp and others have some small percentage of 3 eyed blood, which allows them to use VOAT?
its not impossible to think that 3 eyes intermarried with members of the D clan, as both were persecuted by the WG.
In any case I think this could be the basis for a power up for Luffy. 3rd eye awakening.
Yes Luffy does not have a literal 3rd eye. But in Hinduism and buddhism, the third eye is invisible. Opening it is associated with enlightenment. https://en.wikipedia.org/wiki/Third_eye
It is associated with Shiva, a god of destruction like Nika.
I think Luffy might fully awaken his 3rd eye, giving him the power to fight Imu.