Before I begin, I suck at art, i'm no good with design and i couldn't make a TF2 item if i tried, but... I do have a fair share of Ideas. Since our Engi has his nice little Hard Hat and Safety Goggles to protect him from Workplace incidents, Why not complete the Safe on the Job site set and give him his very own Fluorescent Vest. The Lighter yellow would be used for the Red team with a more green approach lined up for the Blu's. This item would not be paintable, but if needed the trim could be paintable in the wide arrangement of colours available through TF2. Obviously in the image i have used flat colours but the reflective strips on the vest would have a shinier aspect to them. (Pretty Sure this wont make the cut Badge Wise but I hope feels good to share some ideas and make a contribution.)