If this was a system, it would mean boss fights will be a lot more phase based, and there wont be any "dedicated role" anymore.

What I mean is that you wuld likely swap from one roll to another depending a part of the fight. So if its Destroy this Crystal, or Boss doesnt have an aggro table and attacks randomly, you would swap to a class that is DPS from lets say Tank or Healer. Or The boss might have tons of Tank busters, so now you need DPS to swap to tank and take over when teh Initial tanks run out of Cooldowns. Itd be an interesting system, but also know that would be extremely hard to balance and likely a s-show to implement. Not to mention, that means either dumbing down the classes even more, or requiring everyone to essentially level all their alt calsses to max, gear them to max, and then expect them to all be played at nearly the same level to do any end game content.

That would be a nightmare for a lot of people.