Magic Immunes would be a good idea, though might think about Lower Resist affecting magic res again (already saw a post about this somewhere).
The Bonecro isn’t a strong PvE build and while Magic Immunes would help reel in the Hammerdin a bit, it might be to hard on the Necro.
Iron Maiden concept sounds great.
Mana Drain yes sure, however I think it should be based on % of max mana, otherwise it will either drain a barb instantly or a sorceress not at all.
Shrine spawning a super unique only if it doesn’t become a thing to hunt for the great drops.
The drops should not be worth hunting these, it’s fine if the drop chance is worth popping one if you encounter it of course.
Always for making the game harder.
Maybe random event like encounter, darkness falls (like in The Tainted Sun) and you hear Ahh, Fresh Meat
Kill what’s coming for you to end the darkness, might have something interesting in the drops, but nothing brilliant enough to make you want to kill it.
Going to town might end it as well so people could choose whether to fight it or not.
Personally I thought the inferno diff was great, definitely not a walk through the park, but that made beating it feel a lot more rewarding.
Basically the opposite of what it is now.