In a recent interview (which ill link below, its worth a watch imo) a dev mentioned that he felt bosses are too easy in their current state, which I agree with. He mentioned adjusting the move sets on bosses and enrage timers as possible changes to address this, and I personally am all for it. But what if we had elite bosses that could only be summoned after defeating the 5th boss? The idea being that you would NEED end game gear to stand a real chance, and it would be a challenge for players. I do realize that this is a LOT of extra work, and that it also may wind up feeling a little samey or grindy to fight Eikthyr 2 but this time its harder, but I personally would love a little more challenge and some things to really use our high level gear on. And ideally you wouldn't just increase the HP pool, you would maybe introduce some new mechanics to the harder fights.
Link to interview: