Me and I think Clefairy were rambling about a "backpack" unlockable a while back. It would replace the shotgun but allow the Engie to carry something on his back, while still fully operable.
Buildables would slow you down depending on what it is and what level it is, too. Level 3 sentry? You're a walking megatank that's slower than a Heavy.
Also, I found these while I was at Anime Expo last weekend. Thought I'd like to share.