I'm just not buying the whole "tinkerer" concept...
If there was such a thing as a "support" class, than sure.
Not to mention it TOTALLY clashes with an actual in-game profession of "Engineering". One would say "complements" - but IMO that leads to a possibility of having a class that DEPENDS on maximizing itself based on one specific pre-determined profession.
But here are also the practicalities of adding in a new class
1) adding yet a WHOLE new set of talent trees and abilities that will FOREVER be tweaked in balance with 11 existing characters.
2) You're now FOREVER committed to making a 12th PVP set and 12th Raid tier set design from now until the servers shut down.
3) You have to make the gameplay ORIGINAL enough to feel separate from every other class while still trying to make it fun.
A fourth spec is far more likely than a new character, especially considering how simplified the specs currently are. Who lot less balance issues you have to deal with and a whole lot less future work.
...and that's EVEN IF they do a fourth spec.
Chances are that the easiest solution would be to simply add in a new "neutral" race and call it a day. Only thing you have to worry about are starting zones, animation and racial traits. And when they're done - THEY'RE DONE - no more going back to them.