And also some of Monks' space.
Rogues' smoke bombs. Hunters' traps. Hunters' mechanical pets. Engineering toys and bombs. Hunters' guns. Gonbins' racial leap. There's plenty of stuff that they'll overlap with.The Tinker won't have that problem. They won't take abilities from existing classes like Demon Hunters did, and they're not another agility-based melee class. They'll be ranged and use a theme that's largely absent within the current class lineup.
And then there's the matter of niche. There are already a lot of pet specs - 2-3 Lock and 2-3 Hunter depending on whether the pet-less builds are available and viable plus 1 Mage and 1 DK gives 6-8 perma-pet specs, 4-6 of them being ranged. Then there's Elemental and Enhance Shamans and Shadow Priests with short duration pets - and Shamans are getting turret totems back on top of that. Thus, they probably shouldn't be a pet-spec, but there are also lots of non-pet ranged.
Then there's how they do their DPS - single-target or AoE? There are plenty of specs for both. Burst or sustained - again, both are filled. DoTs vs direct damage - again, plenty of both. Turret vs mobile? Same problem. Blizzard already has trouble making useful distinctions between specs and classes now.
Maybe they could heal, but we've got a shield healer, and HoT healers, and direct healers. We've got ranged only healers, and melee healers. Maybe they heal and do damage via turrets and golems? Ah, but we have pets and we have totems and statues for these already, so the 'Tinker' would stomp on pet DPS and on Mistweavers and Resto Shamans and also over the part fifteen years totems have always had a mixed reception and that won't change just because they all like Goblin totems, or run into melee and blow up in a burst of healing goo.
There's not a lot of design space left to put a new class into, and players will be really pissed if its carved out of their favourite class the way introducing DHs chomped off a big chunk of Demo (and put paid to any chance of a Lock tank build).
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Druids already do. Paladins could simply by adding a spec that does ranged Holy damage (and there isn't a ranged holy DPS, so that's an open niche for a Paladin or Priest spec). Shamans used to be able to tank non-raid content, so there's precedent for them having a fourth role. That's three classes that do, did, or easily could fill all roles, and they are in the game right now.
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Dwarves make and use tanks, aeroplanes, and artillery, and of those only the planes might be borrowed from the Gnomes.
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They might be a decent representation of the population that's "members of MMO-C that answer polls". I think it's a bit less likely that they represent "members of MMO-C", and I very much doubt they're representative of WoW players as a whole.