Another speculation is that Witch is related to "breaking the rules" of their aspect. Feferi was able to establish an afterlife through dream bubbles, therefore breaking the rules of Life. Jade displays powers that completely go against the laws of physics.

I mean, God Tier is literal Godhood, I’m pretty sure every Aspect is shown violating some sort of rule of nature, from Resurrection to Creation out of Nothing, so ‘breaking the rules’ isn’t really exclusive to Witches.