I think a Tinker class could really work. It would allow for a ranged, melee, and even tank build quite easily. It would work with the lore, and wouldn't be really close to the other classes, like a (dark) ranger would be. Let it drop turrets for a ranged class, and also be able to slink into melee with different attacks. Give it a flamethrower, give it a power fist. If you want to make it a tank, then literally make it a damn tank. Hell, you could even make part of the class specialization that it takes random armor drops, and mods them into machine parts, instead.
Honestly, the possibilities are nearly limitless. It's just a matter of how much imagination and creativity Blizzard has. That's the part that worries me, because I think they will make it bland as hell if they do it.