I think being overly restricted to Pandaren in terms of its design was also an issue. And I'm not only speaking about the visuals and themes of the class, though I think also there just playing the martial arts in a more cool and over the top manner instead of making it look like Timmys Dojo would have helped, also the animations. If you remember, the unique fight animations for Monks looked objectively shit with the old race models which were only replaced on expansion later.
And while DK was absolutely OP during Wotlk, it became less and less so over the course of the game, with Demon Hunter we had Havoc being incredibly strong but I don't think Vengeance was OP for much of Legion. I think its also not just a gameplay issue, its also an aesthetics and accessebility issue. With DK, you could skip the entire tedious classic leveling zones and with Demon Hunter, you could even jump right into the expansion. Not to forget, both classes played on the appeal of roleplaying as the big two of Warcraft. Thats why I personally think just from a pure appeal perspective and what would garner the most play and interesting, Dark Ranger probably is the stronger one, though again, it is always subject to the personal wims and interest of the Devs. I say both of them will absolutely be the next two classes and the only point of discussion is the order.
While you are right that leveling is far easier nowadays than it was back in MoP, I still feel like the best choice would be to make Tinkers start at a higher level and let them skip the tedious leveling, if Blizz wants for many players to jump into the class. Though I will also say, one alternative is to let them start at a lower level but make them available during the pre-patch. I think that Blizz did that was still one of the most genius moves ever. It made people excited to buy the expansion early on and people had the time to try out the class, maybe even gear it a little bit up and don't be forced to decide whether or not they want to just level their main in the new zones or start again with the new class.
To be honest, I was always against your pure Goblin/Gnome based Tinker-Class, but I think the class absolutely can and should have gnome/goblin aspects to it. The best would be to keep the design more generic tech/magitech (and lets be honest, outside of rockets there would be no visual difference between a technological laser beam and a magical one) and include the most wacky Gnome/Goblin elements like mechano squirrel in the form of more gimmicky niche abilities or talents.
When it comes to the name, it depends on how much Blizz wants to stay on formular. Monk is pretty distinct from the old WC3 Brewmaster in concept alone and they still included the Brewmaster into the class as a spec. I would assume Tinker would be included somewhere just because it was a WC3 hero which the class is based on, while making the concept more general.