But its not Agree to Disagree.
Let's look at a history of the classes released into WoW post launch. Most classes covered several of the Warcraft 3 units, which, believe it or not, had its heroes serve as the basis for SEVERAL classes in WoW.
Classes Released Post-Launch
WotLK - Death Knight
Where did we see this before?
http://classic.battle.net/war3/undea...thknight.shtml
MoP - Monk
Where did we see this before?
http://classic.battle.net/war3/neutr...ewmaster.shtml
Legion - Demon Hunter
Where did we see this before?
http://classic.battle.net/war3/night...onhunter.shtml
So, disregarding classes that became NPCS (sorry Sea Witch, Dreadlord, Crypt Lord, Keeper of the Grove, Lich, Pit Lord, Firelord) we're left with a few more options that haven't yet been farmed out to other classes:
Priestess of the Moon
Warden
Goblin Tinkerer
Goblin Alchemist
Dark Ranger
Now, an argument could be made for another ranged petless archer type class --- utilizing Priestess of the Moon, Warden, and Dark Ranger as a base....but largely, the abilities they have ( Starfall, Black Arrow, Life Drain, Blink and Fan of Kives ) have already been farmed out to other classes. Of course, this could be retconned, much in the way Metamorphosis was from Warlocks to Demon Hunters....but we did just get an elven exclusive race.
Now, if a Ranger (petless archer/gunner) class were to be added, there is definitely space with Dark Rangers working for Sylvannas, and Alleria now providing a void option. They could pull new things from there.
But the two units that don't step on any other class's toes that are left from Warcraft 3? Goblin Tinkerer and Goblin Alchemist.
To make it even better, Goblins and Gnomes have bad population representation, and if the class is fun, restricting it to just Goblins and Gnomes ensures a larger population.
Also, the abilities of Cluster Rockets, Pocket Factory, Robo-Goblin, Healing Spray, Acid Bomb, Chemical Rage, and Transmute don't really step on the flavor of any other class. (Sure Chemical Rage is just Bloodlust/Heroism/Time Warp/Ancient Hysteria). But you can also see, that unlike your accusation that this would share too much with Engineering...Engineering provides literally NONE of these abilities. Neither does Alchemy.
A Tinkerer class, using the updated Gazlowe from HotS as a base (since he is literally the tinker counterpart) and including elchemist as a healing spec, provides the last missing piece from Warcraft 3 to be represented into Warcraft. Has nothing to do with agreeing to disagree, when you are just ignoring how Blizzard previously operated in regards to new classes, and what's left from the game that fed directly into WoW's class design.