I already talked about this in the EU forums but it got barely worthwhile discussion material over there, perhaps on MMO-Champion where people are not too bored of reading more than one sentence it will get more attention.

Since the next most possible choice of a class would be a mail and/or ranged class, in my eyes it makes the most sense for it to be the Tinkerer due to a number of reasons:

In my eyes the next Expansion will be about N'zoth, with Azshara being the 8.4 focus or, which I find more likely, Sylvanas taking that place for either a kill-off-sentence or a (I dear god hope not) redemption arc story after defeating her, which leads me to the following points:

Next to magic, the next best thing to deal with underwater zones, possible underwater focused raids, dungeons and what not, is Technology from both Alliance and Horde, with their underwater Boats, teleportation possibilities and other gadgets. Even our beloved Diamond Dwarf still used that sea boat to reach the Chamber for our Heart of Azeroth in the first questline from 8.0.

Both Goblins and Gnomes got a lot more attention in BfA, and even more so in 8.1, with the High Tinker Mekkatorque having to use some kind of freezing gadget to keep himself safe after escaping from a battle with the Horde Players. The Alliance is unaware of how to revert the effect currently, in my eyes it seems logical that this is future content, perhaps for 8.3/8.4 and a new Gnome Allied Race (Mechagnomes), or, what I find likely and possible too, him getting rescued by 8.4 and coming to the conclusion, alongside Gallywix, that both the Horde and Alliance need more Technology to deal with the foes threatening Azeroth.

In WoD, we pretty much got teased each patch harder that the next Expansion will be once again about Demons with Gul'dan, perhaps this is the case with a Goblins & Gnomes focused Expansion, or one where they are majorly featured in?

Next to Kul'tiras and Zandalar, I always also wanted to finally visit Undermine and finally take back Gnomeregan, this would be a good way to introduce both of these Cities with a proper, new shinier look for Gnomeregan.

We are waiting for new Goblin and Worgen Models for quite some time now considering the new character Models came in WoD which was two expansions ago. What if it takes so long because they were waiting to properly build the new skeletons (most notably Goblins) for controlling specific Mech-Suits?

Another small but not-forgetable thing is this: Most if not all enemy Island Expeditions Team you face are based off on playable character Classes (maybe set aside the halfnaked Orc with his Fists), also including Tinkerers/Engineers/Mekgineers in the form of Gazlowe and his Crew, with Gazlowe sporting a sweet mech shooting Rockets, planting Turrets, and having a still some-what unique mechanic where you first have to kill his Mech and then him to beat him, just like most other Goblins sporting Shredders and other Suits.

Engineer/Tinkerer/Mekgineer, whatever you want to call it, might seem stupid to some people considering we already have a Engineering Profession.

However, I always find this argument silly because of multiple reasons: How does Enchanting make sense with Mages and other Spellcasters around, yet a Rogue can do it too? How does Smithing make much sense with Warriors and other Plate Wearers around? Shouldnt they be the only classes for that then?
Engineering is a lackluster profession for a damn long time now, if it wasnt for Goblin Glider and Rocket Boosts and the money-making capacities of the Mounts this Profession would be even more dead than it is.

A tinkerer would be a lot more than just throwing a few grenades and rocket boots. With a heavy focus on Mechs and ranged weaponry, they can have a lot of variety playstyles, with a melee-focused Tank in a more robust Mech (perhaps in the form of one of those Mechs in The Motherlode!!!!, or like Blackfuses Mech) a ranged damage focused mech or ranged weapon spec (using one of those spider crabs like Gallywix in the Goblin Starter Area, for example) a healing focused Medic spec perhaps with stimpacks, beams and other supportive gadgets, and a melee-oriented spec in form of a Shredder like a typical Goblin Lumberjacker or for Gnomes in the way of Mekkatorques Legion Mech, of course all race and different/better looking than their older Counterparts, with Flamethrowers, Saw Blades, Oil & Rocket Barrage, theres a lot you can do with this type of Class.

It could also be the next class with a "shapeshift" system, where your Mech is giving you multiple ways to move around, like Flying Mode with a Helicopter, Fast Travel Mode with either speedboosts or just like the Mech Mounts, Underwater Mode, etc.

Playable races wouldnt be a hard pick, both Dwarves could become one aswell as Gnomes on Alliance. Goblins, Orcs, maybe Mag'har Orcs (due to the Iron Horde, although not entirely related anymore) too for the Horde would be first choices aswell.

And, this is something which shouldnt be fully left aside either: While not really having much to do with WoW itself, there was a lot of attention for Goblins and Gnomes in Hearthstone, with Goblins & Gnomes a few years back and now also the new Dr. Boom Expansion being out. Maybe a little hinter at whats coming next in WoW, considering most Expansions in Hearthstone are based upon real WoW Expansions? (Molten Core, Frozen Throne, Un'goro, Karazhan, etc.)

I honestly cant think of another class but this getting added to the Game, maybe as the final class even.

Only other likely choice would be Necromancer with another Lich King Bolvar focused Expansion.

So far, thats all I can think of for now.

Is my summary too far-fetched?