Here is my latest idea:
Engineer's Tech Arm
If you play Warhammer 40k you know exactly what I'm talking about, if not here:

I thought it would be an epic idea to make a tech arm for Engineer because of the Gunslinger item is an epic idea for him in the first place.
I think the best way it would look is in the middle of the right shoulder blade and spine and would go out to the right side of the Engineer.
[NOTE: To any model makers, if you want to ever use any of my ideas ever, just PM me and we will get stuff figured out]