i'd love to see this in the game, but i think it's very unlikely as a cosmetic. the original tf2 model designers had put a lot of thought into the silouettes that every class makes at a distance, for reasons of hitboxes and making sure you can identify any class instantly. if you add a backpack to the engineer it kinda ruins it. think of how valve allows painting and lots of cosmetic items, but never enough to make you look like a different class.

an alternative and cool way to implement this would be to make it the 4th soldier banner (since soldiers already have the backpack "secondary" slot.as the soldier banner, it would be pretty cool if, upon activation, it turns the soldier into a mobile dispenser with a wide radius (like the dispenser range upgrade in MvM). finally a weapon that can refill ammo for others. adds an AOE heal like the amputator taunt? as for balance, i dunno, forces you to equip melee while banner is active? or minus self-inflicted push force (weakened rocket jumping ability)

the soldier idea thing is really far-fetched. it's not a serious idea, but i think valve takes cosmetic slots nearly as seriously as weapon balance. if this was an engineer side-pocket type item like the teddy bear or the blueprints it'll be added easily.

very nice model and idea btw