I loved the thorn, it got me to the lighthouse many many times but it is time to hang it up. It was ridiculously overused in year 1. Your idea for bringing it back is great though. I think the Mark of the Sword Bearer perk would be a little overpowered for PvP specifically(good for PvE though). You said that it wouldn't do too much for PvP as a perk but you also said it's like a mini tether when it comes to damage output so the person being shot would be at more of a disadvantage from the start of the gun fight if said thorn user got the first shot and if teammates were around to help team shot... well there's really no hope. But again great idea.