In D:OS, on harder difficulties (and especially when modifier to be even harder) our main problem was that the tank couldn't die but it really had problems protecting anyone. Shackles of pain and taunt helped, but these were really the only ways to really protect others. I'd love to see more protection-themed skills that really help to build a tank while the others could be damage machines.
Few suggestions:
"Life link - Link with an ally to take 50% of the damage the ally received"
"For the greater good - Sacrifice yourself so that all damage received in an area 2 meters of you will damage you"
"Shield sacrifice - Leave (or throw) your shield on the ground, forming a cover for your allies. The cover has X health and if broken, it'll go back to your inventory with 0 durability. Can be picked up at any time"