While not canon, there is a Star Trek novel, The Sorrows of Empire, that expands on this and describes Spock's rise to power and reign. It describes how he attains power through utter ruthlessness, cunning, and the use of the Tantulus device, a weapon he stole from the mirror Kirk which can spy on and kill anyone, at any distance, through any known shielding or barrier, and with no trace. The mirror Spock works to reform the Terran Empire by first consolidating his power (so ruthlessly he kills his mother to prevent her from betraying him), and then gradually working toward reforms. The book also claims that the fall of the Terran Empire was all some sort of grand plan to sow the seeds of democratic government that would cause rebellions that would destroy all the tyrannical governments he knew would conquer the Empire. He actively sets up an organization in secret bases across the galaxy to some day advance this goal. His plan further shows his ruthlessness because he knows that humans, vulcans, and other races will be enslaved by the empire's conquerors for several hundred years.
So lets pretend that the events of the book are true (I know they aren't canon, but they make for an interesting discussion and otherwise there would be nothing to go on), and Spock, his wife Marlena, and the Tantulus device came into the Star Wars universe around 10 BBY, and Spock spends some time coming up with a plan before he uses the Tantulus device to kill Emperor Palpatine and take over the Empire.
Taking into account how much more ruthless he is than normal Spock, what might happen if:
a) Spock decides to reform the Empire, but the rebellion doesn't know this and opposes him, as do some Imperial forces.
b) Spock learns of the future Yhuzan Vong invasion (Outbound Flight suggests Emperor Palpatine did) and decides the Empire may be necessary to resist them, so decides to put off fully reforming the Empire to maintain a militarily strong state.
All Hail Emperor Spock!
(that would be so cool to hear in a movie)
