A Google search was required to ascertain who Grand Moff Tarkin is, but an origin story might not be bad if written and directed correctly. Origin stories can often be interesting if not done solely for extra income. Wayne Pygram is a talented actor who could play the role well, if chosen.
Grand Moff Tarking is a polarizing character, and has been called one of the 'most formidable villains' in Star Wars history. So maybe his origin story would make an interesting movie. It might be more of a challenge, since the Star Wars universe revolves around heroes, but you can't have them without villains.
Grand Moff Tarkin, though important to the plot line of "A New Hope," is not a significant enough character to deserve an origin story movie. perhaps his character is more fully developed in the newest Star Wars film. Nevertheless, a film related to this character would need a significant plot twist to justify a whole movie on a character such as this.
Ever since Disney bought the Star Wars franchise, there has been story after story. Disney knows a money maker when it sees it, and they have just been churning out episode after episode. It's hard to blame them for striking while the iron is hot. Tarkin would be just another story, and not interesting enough to have his own origin.