Out of all the professions the engineer seems to have the least customization for their weapons. Everyone else can display a back item with amazing weapons proudly without it being covered up by a weapon kit.
So My suggestion is to give change some of the weapon kits with different back items to give us a bit more style. I often find myself getting jealous of some of the enemies. Especially the flame legion charr with their awesome looking flamethrowers and back piece. I would like to have those options.
It might also be cool to have a legendary back item that changes all our kits in some way. Maybe it would be a bit harder to implement. But come on. it would be awesome. Imagine a TF2 reference with the flamethrower spraying bubbles.