Expansion classes are tied to major lore figures, and Blizzard has used WC3 and HotS to base abilities for those new classes. Gazlowe obtaining HotS Tinker abilities in WoW is a huge step towards a possible Tinker class inclusion. I dont see a situation like that for the other class concepts you mentioned.
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But we know that Goblins can put entire towns inside a box, so obviously they have the tech to put a mech into a backpack right? Since we know that Goblins can put towns into boxes, what stops a Tinker from simply having a backpack to carry his or her mech in? That could in fact help explain where the mech goes when the Tinker isnt in combat mode.
Essentially all the pieces are in place, just some minor details have been altered to make the concept more acceptable to a wider audience.