Some of these perks rival exotic perks, which is entirely overpowered. If something like this was implemented, the perks would have to be small enough to balance, but large enough to feel like they are making a difference in gameplay. Since the main function your ghost serves is reviving you and opening doors, the shells would likely have perks associated with those activities. Providing a 15 second over-shield immediately after activating something with your ghost seems more sensible to me. Logically, you wouldn't have room to store an extra grenade on a ghost (Unless you can use your ghost as a one-time grenade and be rid of him completely.. Which I'm totally for).