Without thinking too much how this would work or be balanced in terms of WoW gameplay (as we’re talking about RP primarily), I’d split the focus on Tinker into 3 “specs”.
First one is a Tinker that drives a vehicle, inspired by Warcraft 3 Goblin Tinker’s ultimate spell. The Tinker would “transform” into the vehicle form similar to what druids do and would use the abilities of the machine. You could use Warframes, Shredders or mechs Gelbin and Gallywix had in BfA as basis in terms of looks, distinct for each race. You can shoot rockets, drive over people. A unique mechanic might be that, once you lose the vehicle in combat (a.k.a. get to 0 HP), you eject and need to rebuild it, becoming extremely vulnerable and without abilities.
Role: Tank (duh)
Second is inspired by base model of Goblin Tinker: You have mechanical arm that acts as a weapon. Shoot lasers, throw grenades, use it as a flamethrower. Kind of the basic fantasy of being an engineer that has empowered themselves with machinery. Pocket Factory could also be an ability of theirs.
Role: Ranged DPS
The third would be a combo of placing turrets and having a mechanical pet that fights for you. Unlike the previous two, this Tinker doesn’t fight by themselves but rather sends the “pet” after their target while placing the turrets (the spells this “pet” does would be part of the character’s spellbar if we’re talking about the game itself, not RP).
Role: Ranged DPS
In terms of races: Gnomes, draenei, void elves, mechagnomes and Lightforged for Alliance.
Goblins, blood elves, Nightborne and Mag’har for Horde.
Gnomes, goblins and draenei are obvious, I’d hope. Mag’har are there because Iron Horde and its machinery. Elves have their constructs, but on top of that, Sunfury had plenty of engineers at Manaforges and could share this knowledge with other belves, which would then translate to void elves (could have been some Sunfury among them) and Nightborne (their closest ally) learning these skills too.
I’m at loss for what their resouce might be. Not sure if they even need one outside of additional stuff such as Holy Power, Soul Shards or Runes.
Perhaps something like Energy Cells, to fuel the bigger spells, while the ordinary ones work based on cooldowns like they do for Retribution paladins.
Armor type: Cloth, to go with the vibe of “it’s not the Tinker themselves that fights”. Also, Intellect is already the stat there, no need to rework old items.
Weapon: Wrench, a new one-hand weapon type. Off-hand item in second hand.