I would bring back dps stance - shape or form doesn't really matter to me.

Could be a slight dps buff, with a slight mitigation nerf, or a huge one which makes you as squishy as a dps - best example that comes to my mind is the essence/spirit of the irregular from Bozjan.

A big issue I see with the old tank/dps stance:
Tank stance was never really a gain for YOU, I would even question it being a gain for the party outside of ARR. In terms of suvivability and dps output the trade was pretty even, e.g. on PLD you lost 15% dps, but gained 20%, net loss/gain was pretty much zero!
Tho, pre-HW (3.2) the damage reduction was set to 20%, which was undoubtly a loss for the tank.
But the real problem arises once you take dps stance into account; it had no drawbacks; a pure net gain of at least 10% in dps. And why would YOU have to suffer, the tank! Tanks a few, you are the most important person in the party, dps and healers adjus-... ahem, anyway you get the picture. Additionally, 2/3 tanks had to spend a whole GCD to swap stances, so it was WAR pulling 99% of the time. In a DRK/PLD scenario, you better had a NIN shadowwalking you!

I suggest a small dps increase of ~10%, but it negates the tank mastery damage taken effect.
This means, you may gain 10% dps, but lose 20% mitigation, so you take 25% more damage. e.g. DRK gets the action Darkside with 15% dmg increase, but no MP regen ticks and 25% more dmg taken, WAR raw 5% dmg and 10% crit hit rate, but 20% less HP etc.

These stances could get restrictions, just as they cannot be used in Ultimates, maybe even Savage. They should be available to every normal raid tho (8 and 24 man), even dungeons. Tank busters a far and few in between and a good tank can make a big difference there. And the action being oGCDs would also make them convenient.

We could go pretty hardcore and add a stance comparable to the Bozjan irregular potion. Not available outside of solo duties or the overworld (like FATEs and such), as you will become as squishy a melee dps with some mits, but you might deal as much damage as a ranged dps; and enmity tools cannot be used during this stance.

Just some thoughts.