I suggest making an ability called "Grit" which increases enmity generation by 20% and has a recast of 10s."This guy is crazy!", I know. Hear me out though because I think this benefits everyone while solving some issues.
How it helps all tanks:
By no longer affecting damage, this will make it easier for Off-tanks(Side-tanks) to maintain their position of 2nd in the hate list. Tanks can also have their tank stance on prior to tank swaps without worrying about losing out on damage.
How it helps WAR:
To accommodate this change, WAR will need to have access to all their abilities at all times. I think this will be a major improvement to the job and will shift the focus on the Beast Gauge meter accumulation.
Currently, a big focus on WAR is switching back and forth between stances, and it's a bit unfair for WAR to have its highest damage attack and most essential damage mitigation locked behind separate stances.
How it helps PLD and DRK:
By removing it from the Global Cooldown, PLDs and DRKs will finally rejoice in that they no longer have to deal with the issue of their tank stances on GCD.
There will need to be some adjustments to facilitate these changes in regards to Sword Oath and Deliverance. Either make them function like Darkside or remove them altogether and balance damage output accordingly.
No one enjoys having to wait a GCD to use a tank stance and having to wait another to resume their rotation.
How it helps the entire community:
There are players who either want to do more DPS at the cost of tank stance, seeing it as useless, or players who are more concerned with surviving and would rather not be pressured into dropping tank stances and putting any extra burden on healing. By removing the damage penalties, it eliminates the concerns from both these sides.
Furthermore, healers will no longer have concerns regarding this if all tanks take the same base damage. Defense values can be scaled accordingly to fit the loss of damage taken. While this sees Offtanks(Sidetanks) being buffed in terms of defense, I don't think this is an issue.
How it helps the Developers:
Predicting outgoing damage from tanks will be immensely easier. The dev team will no longer have to balance the tanks' DPS around two stances, which frees them up to focus on other things. I see this as one of the greatest benefits, as it leads to more efficient developing which will benefit everyone.