You're right, there's no way of knowing -- but we can make educated guesses about what most
probably won't happen.
The DK was far-and-away the most requested class in the early years of this game; yet, despite being almost exclusively built around the Humans/Dwarves of WCIII: Frozen Throne, it didn't really affect the number of people playing Humans or Dwarves at all in any noticeable away. As you could verify yourself, 16.5% of all U.S. characters are human, 16.6% of all U.S. Death Knights are human. As well, the addition of DH's didn't noticeably increase the number of people who play Blood Elves or Night Elves.
So the question I'd posit is, if the DK didn't increase the overall number of Humans/Dwarves to any noticeable degree and the DH didn't increase the overall number of Night Elves/Blood Elves to any noticeable degree, why is it that you think Tinker is going to somehow double, triple or even quadruple the Gnome/Goblin population which is what would need to happen for Tinkers to even reach parity with what is currently the least played class? Curious.