In Log Horizon, there are moves which make taunts in PvP very effective, this type of thing can also be seen in SWTOR as well.
If you do "not" attack the taunted target in Log Horizon, you are immediately damaged each time you do not target the tank, also it forces you to target the tank at the start.
So for example, I think "Provoke" needs to function like this:
Provoke:
Forces you to target the tank for 1 second. Each attack made against targets "other then you." are retaliated dealing 150 potency in physical damage to the target. This effect lasts 40 seconds. Provoke should be "ATTACK ME, IF YOU DON'T YOUR GONNA GET HURT!"
(note: the tank would be bright red or have a line indicating hes provoking you.)
Bodyguard:
A tank skill which all tanks in PvP should get, which absorbs 30% of all the damage the target takes and raises the targets damage reduction by 5%. This effect has a short range and makes a "chain" between the targets and the target being body gaurded. So you know that the tank is absorbing that targets damage.
Enmity attacks, or attanks in tank stance:
Enmity attacks should lower damage against targets other then you by 25% for 1.5 seconds per enmity attack, all attacks in a Tank Stance are enmity attacks. Each hit refreshes the duration back to 1.5 seconds so the tank must be constantly attacking to maintain this effect.
This would give tanks a viable role in damage reduction for PvP and would also make use of all the defensive abilities tanks are meant to have. Tanks have had their damage cut by a big chunk and have no real role in PvP right now.
A tanks role is to protect their group and be the "first" ones to die, however with current threat mechanics this only works in PvE and doesn't translate well at all in PvP. They are the "last" ones to die because they are the weakest of the group, healers right now forfill the Tanks role.