I think OP we don't need more difficulty but more engagement. Hear me out:
The dungeons stopped teaching mechanics and asking the player to do them and even when a mechanic happens it is without consequence or meaning AND worse many times without proper telegraphing that something happened.
Let the trash Mobs teach the boss and its theme. Then in the boss arena get a last brief show of with consequences. Let the bosses be bosses and make mechanics that need movement or a specific behavior a normal thing with easy to see indicators and teach them early on.

Let the player or the whole gropup interact with the boss and the arena. Also STOP MAKING SQUARES/CIRCLES FOR THE BOSS ARENA. Involve the lore make it so we think it is alive and in the world aka part of the world (smoke and mirrors). Nothing what i propose needs a difficult increase.
That would be fun and enganging. In the end its MSQ and should be easy but not void of interaction or consequences.