What I want and the reasoning for it touches on many of the things commented here already, but @Haxorzist’s ideas were closest to mine for now.
I think that the damage breakpoints should stay the same as they are currently on Legend (or very close), as while shifting them would certainly change “the meta” somewhat the most significant effect would be of obsoleting half our current weapons, and practically forcing us to reroll everything. As @Kekmaster said, making the enemies even tougher (and harder-hitting) would also likely move the most effective stats even more towards just getting as many hits out as you can, obsoleting even more Properties and Traits.
And as some others said, just increasing the enemy amounts is kind of out of the question too, as it would increase the hardware requirements significantly. I also feel it wouldn’t really do anything that could be done otherwise, and might indeed make the game more tedious as all the challenge we’d face was more of the same.
As for making the enemies hitting harder, well, several enemies’ attacks can already down a character in one hit on Legend, and getting more attacks to that category would make the gameplay quite unforgiving - and reduce the significance between different amounts of base Hit Points or Damage Reduction, quite easily to the point where either would be moot for the lower differences. It also would make the gameplay into a no-mistake one, punishing every mistake horribly and likely staring a failure spiral very easily. I don’t think that’s good design in general, and especially I don’t think it’s a fair way of making things difficult.
So what would I like, then? First, enemies’ mass could be increased, effectively reducing our stagger and cleave. This would likely make the hordes feel larger as we couldn’t butcher them with a few swings, and lower-stagger weapons would be more dangerous to use, requiring more care. I think it would also shift the effectiveness of weapons with different amounts of cleave and stagger, still letting us (or making us) learn some new things. Second, the intensity of the action could be increased. For now, we have pretty long intervals between hordes and waves so there’s plenty of room to advance and relax, usually an area can be cleared of Elites (and others) before a horde arrives and pulls them in, and even Specials generally come in very manageable numbers. Increasing the roaming enemies and Specials (particularly the maximum amount out at once, but also the frequency) and decreasing horde timers would already make the game significantly more intense, especially combined with the increased danger from stagger and cleave resistance.
Also, like Haxorzist said, tightening the margin for certain things (dodge timing, primarily) would also add to the skill requirement and danger of certain enemies without adding to their sheer lethality. I don’t quite agree with removing temp health Talents, but their effectiveness could certainly be halved or so, making us rely far less on temp health but still allowing its use (and not just blatantly disabling our Talents).
And yeah, less resources to use, including reducing the end-event preparations to topping off two or three characters instead of everyone, would also be a thing to do.