Every WoW expansion eventually veers back into cosmic powers regardless of how grounded it appears. The most likely Tinker expansion revolves around Undermine. Undermine was originally conceived to be a continent in Vanilla WoW, but developers felt it would be too much work to put it into the game, so it was scrapped early in development. However, throughout the history of WoW it is brought up frequently in a variety of quests and NPCs who supposedly come from it.
You could have a WoW expansion start off in Undermine and over the course of the expansion other things pop up, such as
"The War" the mysterious mechanical war taking place beneath the surface of Azeroth. You could also have a continuation of the Heart of Azeroth, since that's also underground. Within the Undermine lore itself there's the story of some sort of creature within Undermine that is believed to be an old God. Finally you always have the potential to encounter some sort of Titan facility beneath the surface of Azeroth. We could also slap on the retaking of Gnomeregan as a raid, bring in the expanded Gadgetzhan concept (first seen in Hearthstone, but there's no reason to not bring it into WoW), and maybe explore some nearby islands to Kezan like Tel'Abim.
Personally, I would like to see WoW deal with a mechanical threat like you see in popular science fiction. Terminator, Irobot, and Age of Ultron dealt with this concept, and it would be interesting to see WoW deal with it as well. Especially when you had glimpses of how destructive it could be with King Mechagon's device and MOTHER. You could tie the antagonist in the Blingtron war (IR-to) to this concept and make it a major antagonist.
In short, there's a lot you can do with an Undermine expansion, and still bring in the bigger cosmic threats.
Certainly, but the WC3 hero gave them a starting point to work off of. For example, look at how Brewing became a major aspect of each spec in MoP. Each spec had a system for brewing a unique potion that they could use in combat. In addition, multiple brew abilities and talents were given to the class. Not to mention the culture and style of the Pandaren race permeated the entire class. This gave the WoW Monk a very unique feel that a lot of people really enjoyed, and really made it feel different than a generic RPG Monk you see in other games.
Look at the situation with the Tinker; the class already has a good set of unique abilities and attributes. Not to mention that we even have a lore hero to focus the class around when it does enter the game. That solid foundation comes from using WC3 (and increasingly HotS) as the foundation for the classes.
Blizzard doesn't have to use WC3 as the foundation for their expansion classes, but I'm not seeing a reason they would deviate from doing it when it's been highly successful in the past, and they have a robust class concept from WC3 currently waiting in the wings.
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Keep in mind, it won't be the SL team doing Dragon Isles if its the next expansion, it'll be the team who did Kul'tiras and Zandalar, which I'm fine with. I loved both those zones.
As for the possibility of a Dragon-themed class; I'm definitely for it if it allows us to actually be dragons. I have a sinking suspicion that instead we won't get a new class in Dragon isles, but we'll get more covenant style stuff to match up with the dragon flights. That honestly mirrors how Dragonsworn behaved in the old RPG game, and I could see Blizzard going that route.
And it would suck. People want to play as dragons!