I'm thinking Dark Knight's rework will be more extensive than I initially thought.

[Phoenicia]

From a base concept, sure. From a mechanical implementation, not so much.

I honestly do hope it's something along the lines of Ammo Arts. From a flow perspective that alone can differentiate Gunbreaker, then make the stance flavor being the type of Ammo they load.

Tank stance ammo being more point blank effects with heavier expenditures while the DPS stance ammo is more variable (Cuz uptime). A slower bulwark style when tanking with point blank Megumins while a more mobile, all-range capable fighter when not.

With the way this game's "meta" works (encounter design and patterns), it is bad to lock abilities behind the stances of a tank, just look at WAR and its Inner Beast vs Fell Cleave (Guess which move is considered to not exist at all?). DRK's Blood Pact and Blood weapon, anyone? BS that PLD doesn't have to deal with at all which makes PLD a lot more comfy.

While the concept in itself might seem interesting, the game itself doesn't lend to such opportunity cost play-style. There is little reason to use Inner Beast when Fell Cleave exists. The mitigation is not needed because you have a gazillion other CDs to use.. And holmgang. Due to the game's nature of predictable damage and emphasis on DPS, if you remove some of these CDs or nerf Holmgang in order to force WAR to use Inner Beast, it will be shelved and the other 2 tanks will be favoured since they do not have to deal with stance-locked BS. I call it BS because, while cool in concept and identity, it is merely a hindrance and/or clunk from a game-play perspective.