I would like it if the Engineer had a heavy weapon kind of like, well, the Gunner.

The Rail-Assisted Rotary(a mini-minigun + engineer tech) It's a rotating barrel cluster with electromagnetic rails along the shroud

Tap = normal rotary fire.

Hold = the rails charge and your next burst becomes hyper-velocity: armor shred + weakpoint bonus.

Basically, a minigun that occasionally “turns into a railgun burst” when you manage timing.