[SolAndLuna]

To be honest, from my understanding, all of the advanced classes are basically meant to be a whole new class with new perspectives, with only one key thing connecting it back to the base class. Spellthief and Void Assassin are supposed to rely on deception (illusion spells, some of the dagger skills for Spellthief) and underhanded attacks (Cutthroat, basically all of the void), the essence of a "rogue" kind of character. On the other hand, Engineer loosely fits the kind of a different kind of "rogue" character - someone that gets other things to do the job for him, while being ready to throw down himself if the need arises. (that and throwing bombs and traps around generally sounds like a pretty rogue-like thing to me at least) Similar stuff comes up with things in other classes, too. So, I think that the class makes sense from a sort of character building perspective, it just mechanically doesn't quite connect with everything else aside from the Wind buffs it can provide.

Aside from that, very few promotion classes actually have a sort of "no magic, no special power, just skill" kind of thing, I feel. Kensei and Arbalest are about the only ones I can think of off the top of my head. So, it probably wouldn't be bad to have something that gives off the same kind of feeling in a class. I just disagree with the idea of messing with existing builds for that to be done, really, that's about it.