Whatever class we get next will probably depend on the theme of the expansion we have.
Tinkerers would probably come with a mech-type theme (like we’re fighting a technology-based enemy).
Necromancers would probably come with an undead theme (like a return of the Scourge expansion).
A big theory is we’re visiting the Dragon Isles in a future, if not next, expansion, so we’d probably get a dragon-related class.
Which is ironic because Monk was overtly inspired by WoW’s positionals.
Tinkerers should have come out in BFA due to the heavy focus on tinkerers and mechs in the war effort.
Necromancers should have come out in shadowlands due to the heavy focus on death magic (look at newbie island. It’s all about necromancers).
Yep, they pulled a blizzard and said, “backstab, gouge, and mutilate are very cool, especially how you can waste a mutilate by using it from the front. let’s iterate on that and add side attacks and front attacks.”
Identify what’s good.
Steal it.
Make it better.
The old blizzard way.
Personally wouldn’t like either and would prefer a 4th spec for some classes instead and a 3rd for DH.
I could see Tinkerer being a thing for either Hunters/Rogues and Necromancer as an option for Warlocks. But I still believe Necromancer would be better as a class skin for Demonology Warlocks.
B A R D 
The current team seems preoccupied with originality. Completing missing the Blizzard tradition of doing big, dumb, fun ideas borrowed from their favourite sources and refining it to that “easy to learn, impossible to master” perfection.
Sorry to say, but new ideas generally aren’t good ideas. Mostly because we only think they’re “new” because you rarely hear about failed ideas.
That’s what Unholy DK used to be before they changed it from disease management to another builder spender spec with pseudo combo points (festering wounds). I feel like they could definitely bring disease management back and add some talents that make you focus on summoning more stuff so you get to pick a subspec basically.
We don’t have area denial / control / support specs in WoW so it could easily be a new class that focuses on those.
deploy turrets, deploy shield (barrier like ring of peace), deploy magnetosphere (whirlpool that disarms if you are in it when it ends), drone strike (spender / 30 second cooldown aoe in a line), shocking grenade (ground target stun), scourge grenade (ground target AoE), frost grenade (ground target slow, leaves icy patch), temporal accelerator (area of increased action speed, impacts both allies and enemies, impacts debuffs and buffs (note, this means it could be used to get out of polymorphs faster etc)), temporal deccelerator (same thing but other way around, IE: makes stuns last longer), flame thrower (conal aoe), spider drones (chases, webs, and attacks target, explode at the end of their charge), bolto (single target nailgun shot), deathray (single target channel that damages to user and the enemy), rocket (ranged single target stun, knocks the caster down for 2 seconds, stuns for 5 seconds).
edit: Also, kiss curse isn’t a thing in any of wows classes except for a pvp talent for fury warriors. So having a “do x, but suffer y” would be cool.
Does anyone know why they keep doing this???
That is a design decision. We had support classes with Shaman and Paladin. We had utility abilities that were integral to dungeon crawling. Both of those playstyles were consciously discarded.
What’s the difference in necromancer and warlock, from a practical perspective?
I personally would LOVE a engineer/tinker class. Literally make parts of the engineering profession into a class and can have a melee/ranged/tank/healer. Can heal with tools and turrets, tank with them, or do dps with them too! So fun
Let me tell you. I truly miss support roles after playing the shaman in TBCC. I really enjoy being a person that supports the team 
Both.
We should have four classes for each armor type, each with four specs, for a beautiful, perfect lattice of class content.
Boom!
Yep. The nuance of it all… I miss it. Alas…
That type of playstyle doesn’t seem “engaging” when you can’t show how good you’re doing with meters.
Neither shadowhunter both mail wearer and a voodoo using so it’s the best of both worlds
For sure.
In TBCC, on my rogue, it’s like I’ll start to see something going amiss in a heroic, pool energy, and then kick one, blind the guy in the back, sprint gouge the other guy in the back and all the casting is stopped for a few seconds, long enough for the mage to polymorph and people to realize they need to use cooldowns. Smaller scale, utility heavy game play where the mobs are auto attacking for 6-8k hp (hello heroic BF) is just so perfect. Now, was I doing any damage during any of that? No, I was stopping damage as a DPS because rogue has a ton of CC. That felt great.
Does not compute with modern Blizzard.
You are Rogue.
Rogue is Damage.
Therefore, if you are not doing Damage, you are not engaged with the Rogue class.
Tinker for a number of reasons.
I would like both, but if I had to choose one, I would pick Necromancer. Necromancer is my most wanted class at the present. 
Bards would also be cool to see implemented in the game, and I’d happily roll a few for each spec.