It's a popular fan theory for the next class in WoW, discussed on a lot of boards. The supershort version of the theory is:
(1) WoW classes are based mostly off classes/abilities that appeared in WCIII. Tinker is one in WCIII that is yet to appear in game.
gun toting class would balance things.
(2) In terms of game balance and loot rolls, there are already 3 cloth, leather, and plate wearing classes- and only hunters use ranged weapons. The game could do with another mail wearing gun-toting class- but it would need to feel really different from hunter.
(3) In terms of pure DPS, DPS/tank, and DPS/Tank/gealer hybrid class spec arrangements, the game could do with another hybrid class.
Put these things together, and a potential class which could check all these boxes would be tinker: