It could be something like the engineer starts to glimmer and then he takes out a heart and crushes it.

Your sacrifice honors me.

i feel like if he wears that mask he should have special voice lines or at least a haunting like demo's sword.

Just every once in a while youll hear a little bit of crazy/evil laughter (like medic insane) after he dominates someone than a horrified scream from engie's regular voice.

.... it gives the wearer power...a vast increase to his already superior inteligence....but it begins to drive its victims insane....some say its cursed by gray man himself.....others say...its what the administrators first husband looked like before a freak gold smellting and large jewls and crown forming accident.....but now that you say it out loud-

OH! the taunt should be his jaw dropping and then his head spinning around but half way through youll hear a crack and he'll yell "Dammit!" and turn his head quickly back to where its supposed to be.
its both creepy, scary, and funny lol