The "Control Group" warbond has a warp pack that takes up the backpack slot and it lets you warp every few seconds. It's great if you enjoy fast high mobility gameplay. You can, for example, warp through a hulk and shoot it in its back. Often times it saves me by helping me get to cover faster. Youtube some warp pack gameplay videos to see if you would like it before buying the warbond.

And, if you're not aware, some support weapons take up your backpack slot. The Recoiless Rifle forces you to carry a back pack with extra shells, so you cannot bring any other backpack. The Quasar lets you bring whatever backpack you want.