I think the better solution is to not have stances at all. If you try your hardest to force players to use defensive stance through mitigation requirements, there's still going to be a subset of us who manage to get by without it (remember i270 accessories?) So you just create a performance gap.

The take home message should be: stop punishing players for playing the game. People don't like defensive stances because they're effectively just a penalty slapped on to extra mitigation. They're also cumbersome and costly to switch in and out of if you're not playing WAR.

The discussion about MT and OT doesn't really make much sense because nobody really tanks with that mindset in raid content. There are often two "roles", but it's less of "you tank, I pick up adds and afk for the other 8 minutes" and more of "I do these mechanics, you do the other ones".

The instant you say "this content should be able to be cleared by any combination of tanks", your two "OT" tanks are going to be meta.

I'm more concerned about how many times Gunbreaker gets described as a "slashing" damage tank. Please, just kill the 100% uptime slashing debuff. This game can't survive another expansion of forced WAR dominance.