I think the most likely candidate by this point is really the Tinker. I dont see any other class with that many hints pointed towards them being an possible future Class right now than a Technology Class.

If you sum up all the things we know so far and that there is more than likely a chance that we as usual get a new Class every other Expansion we have ultimately still have the following hints, listed up again for clarification:

1. There has been an increasing amount of focus on Goblins and Gnomes in BfA, especially on Gelbin Mekkatorque and Jastor Gallywix, with both and also other less important gnome/goblin NPCs focusing on Mech Combat, which got increasingly more love from Blizzard since Legions Mekkatorque added Mech.

With that said:

2. Gelbin Mekkatorque has survived the Battle of Dazar'alor in a frozen device/state which absolutely no one so far has any clue about, which will undoubtably lead us to Mechagnomes being an Allied Race and them potentially starting in Mechagon.

3. As mentioned by a lot, the Island Expeditions all have a team of three with recognizable playable classes, be it the Nightborne Mages or the (outlaw) orc Pirates. The only exception here are the three Trolls which are all Units from WC3 aswell and one of the Raider Orcs using a barrel and a cannon as fist weapons. Then, of course, you have on each side a set of 3 Engineers/Tinkers/Mechanics, whatever you would like to call them, with all three focused on a different role: Ranged DPS, (Mech)Melee Tanking and Healing. Of course, it does not have to mean anything but considering all the other stuff it adds up alltogether.

4. While long overdue, the finally updated new Models of Worgen and Goblins have arrived with 8.2.5. What better reason is there that it took them so long to just update two Races because they were already internally testing out the new Goblin rig for the new Tinker Class Animations? Might be a little bit stretched, but we got our new Models for all but Worgen and Goblin since WoD, which is now pretty much two Expansions old. Took them quite a while to update two Races, one might think.

What if it took them so long because they couldnt get Tinker to work in a Legion Expansion (where they rightfully added DH and was their only way of doing so) so that they waited a little while longer so the Goblin Animations would be perfectly made inline and adjusted for their new Class in 9.0?

And imo one of the biggest points:

5. A Tinker/Mechanic/Scrapper/Siegecrafter, whatever you would like to call it, has more than enough existing Content for all three roles. Not only that, considering our latest Class only got two Specs, what if the next upcoming Tinker Class is gonna be our second Class with four specs, being able to do both DPS roles aswell as healing and tanking? I know Blizzard said that they would remove a few specs if they could, but that is because of those Classes playing more or less equally despite different specs, an issue this Class wouldnt have.

It would be fairly easy to implement as a Tinker in comparance to other potential future Class ideas like Necromancer, which more or less heavily ties into with a DK already or Dark Ranger which straight up doesnt even have two explainable specs right now. A (dark) Warden has absolutely nothing to do with a Dark Ranger and it wouldnt make sense for them to be one class combined either.

Coming back to the Tinker Specs, I see the following:

Tank: Riding a beefy Mech just like Mekkatorque, Gallywix, Blackfuse, Thermaplugg, etc. etc. with energy shields, mech plating or even short-lived summoned Bots taking some damage/taunting away, quite like Brewmasters Statue.

Healer: Using a wide array of different healing Bots providing AoE burst healing, temporary AoE healing, Healing Sprays/Guns/Elixiers and added ranged weaponry.

Now this is where it gets interesting. In my eyes they easily can make both a melee and a ranged spec without harming the other.

Melee DPS: Riding a Mech aswell, but this time focused more on combat than defense, including Flamethrowers, Saw Blades, using their modified Mechs Arms as Guns/Cannons/Weaponry, Rockets, the list goes on.
Ranged DPS: As speculated by others already, a ranged non-mech damage dealer with a focus on different gadgets, damage and CC bots, turrets, grenades, maybe even pocket factory and their gun.

How people think that this has anything to do with Engineering tossing a bomb all two minutes is beyond me. Feel free to dislike the idea of this Class, I for one dislike big Tauren Monks doing flips and hurting people with hoofkicks, but that doesnt mean that they have no reason to be there.

To keep the Class identity in check (which Blizzard apparently wants to get back to instead of Spec Identity) all of them would have some stealth gadget one way or another (potentially even leading to another Stealth AoE like Rogues have it), and some bots and gagdets for CC and healing.

My last point would be that there is no better time to add Tinker than it is now. Either they come with 9.0 or they unfortunately wont come at all.

We have people getting used to seeing Goblins and Gnomes running around in Mechs, and even have an extremely cool Bossfight with Mekkatorque in his own mech.

Adding onto my second point, with Mechagon in place for a starter area for a (hero) Tinker Class and Mechagnomes and Vulpera the most likely candidates for 8.3/9.0 Allied Races, they're opening up two more small Raceslots for Mech Combat while also adding a rather popular race into the Game. Mechagon could also serve as the future starting hub for the Tinker Class, quite like Acherus does for DKs and Mardum does for DHs, so the pre-purchase of the new Expansion would be perfectly fine then aswell for them to be playable in 8.3.

If we really get a Shadowlands/Old Gods Expansion, technology would be a pretty powerful tool to combat them. Things like that Shadowlands Leak with Tinkers creating a device to slip in and out of the Shadowlands? It doesnt seem too farfetched considering they are brilliant geniuses, or them using their advanced technology to avoid getting mind controlled by the Old Gods Whispers.

I really hope we get the Tinker finally, but everything can happen. Would I be mad about a undead Human Dark Ranger like Nathanos if the Class is cool? Nah, not at all.

They also potentially can pull out a completly new Class which no one or very few have expected, quite like the Dragonsworn or Aspects, which also would be able to three role and even offer up to five Specs with each Dragonflight.