they already tried everything...
big boss with complex mechanic and multiple parts that you need to destoy first to kill? eidolon and those fortuna giant spiders, ppl just speedrun and oneshot them even in sp.
bullet spongy boss? archon, ppl also just speedrun and oneshot them.
boss that can oneshot you? revenant.
you need to realize that the only way they can make a challenging content is by nerfing the players to the ground, which is just never going to happen.