I think that to fix tanking stances they should just delete every drawback they have. No damage decrease, no ability locking (Fell Cleave, Inner Release, Blood Weapon, Blood Price). Tanking stances should just give the %mitigation and the agro, and be off the GCD with no cost, and DPS stances should be deleted or turned into passive traits. That way, the only reason to turn it off would be not to accidentally steal aggro when you're OTing.

Why am I thinking that way ? Because the "aggressive tanking" meta has changed. Before, back in ARR, the "aggressive tanking" meta consisted in proper stance-dancing which required actual skill to be properly achieved. We were switching between stances because we actually needed them sometimes back then. WAR for example didn't have a DPS stance, they didn't have FC, and they didn't have Rampart, Raw Intuition nor Equilibrium. PLD didn't have Sheltron and Clemency. Tanks back then basically had less defensive abilities to compensate for the lack of tanking stance, and WAR actually used to not lose any DPS by switching back to Defiance for 20s or less when Unchained was available (it was even a DPS gain when combined with Zerk, since IB was the hardest hitting skill, and being in Defiance was the only way to build stacks). So we used our tanking stances for tank busters and other big damage phases. Only Rampart or only Sentinel weren't enough for big tank busters such as Ravensbeak, Revelation, Flatten or Akh Morn. We needed those extra 20% from Shield Oath. We needed that IB on top of Vengeance and the extra eHP from Defiance. "Aggressive tanking" consisted in optimizing both your cooldown usage and the moments where you'd switch back into tanking stance because you needed it in order to gain DPS. It was a careful task. It added a very interesting mechanic to tanking.

But it changed. Partly because of encounter design perhaps, but mostly because starting from HW, while levelling up after 50, tanks gained extra defensive tools which were available regardless of stance, while healers also gained extra healing tools which made the task of keeping-the-tank-alive-while-healing/protecting-the-rest-of-the-party a whole lot easier (stuff like Tetra, Asylum, Emergency tactics, Indom for example; powerful oGCD healing tools). Now, you just never use your tanking stance outside of the first blind runs. You don't even pull the boss with it anymore. You just DPS and use your CDs when needed. Tanking stance is a button that you only press when shit starts hitting the fan and healers need some safety to raise some people. And even in most of those cases, the extra 20% mitigation won't even save you from a wipe. "Stance-dancing" doesn't really exist anymore. It's just an empty shell of what it used to be.

To make the tanking stance and tools like IB relevant once more, they have two options :

1. Delete a lot of the defensive/healing tools that tanks and healers currently have. Basically, make both tanks and healers less powerful defensively. This would give back a lot of value to the tanking stances and IB, since we wouldn't have enough CDs to mitigate everything. This option would change the way we play and might not please everyone since it's a direct nerf to both tanks and healers.

2. Do what I first suggested. Delete every single drawback that the tanking stances have and either delete the DPS stances or make them passive traits. Make tanks able to achieve their highest DPS regardless of their stance. And increase every boss and monster's damage so that the damage we take while in tanking stance is the same as what we currently take without it, that way CD usage wouldn't be trivialized for both tanks and healers. Drastically decrease every aggro multipliers to compensate for the much higher uptime that we'd have on our tanking stance. This option wouldn't change anything to the way we play, it wouldn't make tanking or healing any easier either. We'd just toggle it on when tanking because it's free aggro and mitigation, and toggle it back off when OTing to prevent us from stealing aggro instead of just never using it outside of emergencies like we currently do. This option also fits with what they did to healers by making their damage scale directly from MND, essentially deleting "stance-dancing".
That way, they could also focus on adding complexity to how tanks play by other means without having to bother with the different stances.