I had this idea for an Engie unlock a few days back. Clefairy might remember this.
It's a backpack. It replaces your shotgun, but gives you the ability to literally carry a buildable on your back, completely functional. It slows you down at high levels, though, especially level 3 Sentries.
Like... Heavies-look-fast speed.
Mobile dispenser at the expense of your shotgun? Discuss.