If it were to be an integrated jacket/harness, how about some strong points from which to haul someone back aboard (like a couple of handles). If there were leg loops as well, then an unconscious victim could be hauled aboard more easily.
Extra padding in key locations covered by extra durable fabric.
Pockets that easily drain when filled (I don't need the gear inside to stay dry, but not carrying around extra water is a plus.
How about a removable lining for various temperature ranges.