All old content should get some update to become less stupid. Not really harder, not longer ; just less stupid. It's more noticeable with the crystal tower because it's easy to get it with roulette, especially since item level requirement can be abused, but old contents have become a mess. A boring mess for experienced players, and an incomprehensible mess for new players.

Most boss in the game starts with an introduction phase where attacks are used one after another, and it's only after that that more complex patterns and clever combinations are used and the real fight begins. But since fights become shorter, older boss die before the fight gets really interesting...
It's even worse with boss with special phases that take too much time with the current fight duration, especially when the boss do nothing and can be attacked (Cloud of Darkness)

The boss should be more active faster (for example : Phlegethon should use his bigger aoe right from the start), uninteresting gimmicks should be reworked or removed (Phlegethon flare, clouds phase with Cloud of Darkness, etc.)
For some boss, it can be as simple as increasing the damages (Thordan)