Kali standing on shiva: The dance of kAla (time) and the lIlA of sristi-sthiti-laya-sthiti occurs on the 'chest' shiva who is just a mute witness...mute as a dead corspe.
Buddhist deities trampling on Shiva or Kali is typically to represent the superiority complex of bauddhas vis-a-vis sanatana dharma and typically doesn't have any esoteric significant other than egoistic nastika demonstration of supremacy.