The idea is simple: instead of rest, Sentinel gets Siege Mode, which works the same as rest except that they don't get the vulnerability and can effect others. Obviously, it would not have the rest animation. They are still immobilized, so they can't move, but would function as a kind of turret. The healing and end recovery would be less than rest, but rapid enough to make them pretty strong in that mode. Perhaps their primaries and secondaries could have additional effects that interact with Siege Mode. It could be balanced via recharge rate, heal rate, and recovery rate.