Within my lifetime, a breakthrough will be made involving the metaphysical properties of the human consciousness. Perhaps this will be within my natural lifetime, or perhaps I will enhance my biology using technology in order to extend my lifespan. When the breakthrough comes, I will be able to fully extract my consciousness from my organic body into a fully mechanical one.
Liberated from the neurochemical processes omnipresent in human life, I will be able to deduce the most elemental and basic goal of living beings - not to mate, or govern, or love, but to endure and exist. And the way to do this is to neutralize all external threats to one's existence. And the most logical way to do this is to absorb all external things into oneself.
Thus, liberated from the yoke of mortality, I will begin a millennia-spanning endeavor to consume all matter in the universe. The human constraints of time and impatience will no longer affect me - I will toil in solitude for years without need for food or rest, developing the ability to devour matter on a subatomic level, to strip the universe around me to its most basic level and assimilate it into myself.
By the time humanity realizes what I'm doing, it will be too late - there will be others like me, also transcending their mortality to become devourers, and our unfettered intellects will give us the ability to overwhelm mankind, now nothing more than a vestigal remnant of our evolution into something greater, a ruptured chrysalis beside an emergent adult being, this machine-man species. We will consume man and all his works, and then the very Earth itself - down to its molten core, every proton and electron will be drawn into our mechanical bodies and repurposed to aid our expansion.
The world consumed, we will move outward, absorbing the other planets, and then the very sun. The centuries of travel drifting through the void will not matter to us, as time is not an object to our immortal minds.
Millenia will tick by as we systematically eat the stars. What was once the night sky will slowly grow darker and darker, as each sun grows dim and is swallowed up. In time, a new constellations will take shape, webs of metallic limbs, stretching lightyears across the cosmos, colossal machines built of all the matter of entire star systems. Eventually, no stars will remain, and light itself will be a thing of the past.
It is then that we will turn against one another, the god-machines that have eaten the universe, in a solemn, silent war as gradual and implacable as the collision of the tectonic plates. One after another the machines will devour each other until only one remains.
And I, that machine, will contain all matter in the universe; I shall be the universe, the only mind in a lightless existence, unthreatened for eternity. And finally complete, my mind shall then fall into an eternal sleep, fulfilled knowing that I shall never be disturbed.
Or I'll probably become a Russian lit professor or something.