Whatever the class is it'll most likely:
1: Be a mail armor class to fill the armor type gap.
2: Have at least one ranged DPS spec, again to fill the role gap.
3: Possibly also have a healing and/or tank spec since hybrids are the most popular classes.
With all that in mind plus the fact that there are actual Tinkers in existence in BfA, I would think that it's the next class.
Tinker would wear mail armor and have a ranged DPS that uses guns (and most likely bows/crossbows too, for convenience sake). They'll most likely have a tank spec which has a mech as their tanking mechanic. They'll for sure have at least one ranged DPS spec like I mentioned, that uses guns/bows/xbows.
Their third spec is up for debate. It could be a melee spec since they are still the most popular play style in the game and Blizz would want to pull in the melee players to the class as well. It could be a second ranged DPS spec, since again, they are at a deficit right now. It could be a healing spec in which they heal sort of like Ana from OW with "biotic" shots and grenades. Or they could just have 2 specs like DHs (probably not as likely).
If I had to guess, it would be 1 tank spec, 1 ranged DPS spec and 1 healing spec.