Once a human being reaches state of self realization, god realization the cosmic journey of life finally ends forever. Having become a pure soul atman the need to manifest a new human body ceases to exist. A soul atman keeps manifesting form after another until it contains dross impurities within. State of self realization announces liberation from cycle of birth and death forever.
The status of an avatar is much above that of a self realized soul. Lord Krishna, his soul atman has already left the solar system forever. It is free to venture into any part of cosmos at will.
Lord Krishna was born an ordinary mortal. Seeing the pitiable condition of the world in Dwapar Yuga he took upon himself the task of cleansing the society of its ills. This he accomplished by becoming an avatar of that era. So would be the fate of Kalki Avatar. Unable to bear the torment of masses at large at the hands of administration, judiciary and policing Kalki Avatar would convert himself from an ordinary mortal to almost a super human being.
An avatar does not come from top as many believed. Kalki Avatar is one of us. Because he manifests almost supernatural powers, it is difficult for mankind to believe in his normal self. Kalki Avatar would manifest powers almost like Lord Krishna but would still be even more powerful than Lord Krishna due to warfare of present times and the advanced scientific age.
Only after achieving the unachievable shall Kalki Avatar rest. And then it would be time to bid goodbye to mother earth forever.