What about solo-only random dungeon spawns? Death limit is capped at 1-2 (you can play around with the death limit depending on how valuable a certain unique or piece is from that dungeon), you can get loot in there that you can only get from that specific random dungeon (way harder than nightmare dungeons, harder than Lilith, harder than capstones) so it’ll force people to build around them/acquire items tailored to that boss. Make them have specific weaknesses like dark, blood, fire, lightning, etc.
You could also go further and make some of the unique dungeon bosses invulnerable or vulnerable to slows, stuns, or other types of crowd control. I would like to add that maybe this is a not so good idea for hardcore players to pursue; so it might be a good idea to make these types of dungeons non-permadeath, even for hardcore players!
I also see that a lot of people are hunting for certain affixes, such as vulnerable %, but what if you made some bosses immune to vulnerability?
Additionally, I think I would like the challenge of having a unique boss that can stun or CC through a shout or CC-breaking skill, this might be going overboard, but I think the idea of being forced to dodge will appease some of the more hardcore players in the game (I’m not exactly sure about this idea specifically, though).
I think one of the issues with the game, as it stands, is that people are really focused on tailoring their builds to the meta. But I think this adds a bit of variability so that no specific build will be strong in all areas of the game.
Going to edit this later, apologies for the disorganization, I just wanted to get these ideas down because I thought they would be decent ideas to implement!
I have a hunch that you guys are already implementing something like this in the game, I think that an idea similar to this was already implied by the death limit mechanic in your nightmare dungeons and damage types. But I just wanted to throw this out there, just in case!
Sorry for the double-post, wanted this to gain some traction so that the devs can take a gander at it!