Others have said this as well, so apologies if it's duplicative but I would absolutely consider looking into current extreme and savage content.

Extremes are not the hardest content but they are typically more difficult than a dungeon (unless it's something like Bismarck back in the day ) and savages (especially this tier) offer a good amount of difficulty and coordination. Shiva was a very challenging fight (imo) and I think if you like hard, challenging content, you'd likely enjoy that.

Lastly, there is ultimate fights as well. Dungeons were never made to be for the hardcore. I would love to see different difficulty levels of dungeons for sure but for now, you can't scratch that itch with the available content that I've mentioned above.

Good luck!