The Vest slot sounds fun with the tradeoffs it brings into the game, as currently soldiers of sufficient strength can fill their backpack and carry way too much. Though I do enjoy the belt/backpack distinction where belt items are quicker to access.
What about carrying out wounded soldiers (or captured aliens / items) though? A somewhat uncommon but very cool situation is if you carry a fallen comrade back to the dropship, I've used that in the Cleaner Intelligence Hub to successfully rescue a downed soldier. I hope that system doesn't go away.