I’ll get to your question in a bit, but I wanted to touch on your idea first. While it looks good on paper, there’s a couple problems.
1 - Your idea in particular would be hard to balance. In the case of rare monsters that need a specific gimmick, item, aspect, etc. to overcome, I would venture a guess most players wouldn’t even bother with these enemies and just avoid them entirely. Which was the same issue in D2 with elemental immune elites in higher difficulties.
I’ve always said if the best way to deal with a mechanic is to ignore it, it’s not a good mechanic. Not to mention if you tie special rewards to these enemies you come into the issue we had from the beginning of the game, which is ‘I can’t get these items, fix it’.
While some items should be kept rare, I think the idea you can get any rare item anywhere in the game at any given time is the better design. At that point, it’s just another Lair Boss, only rarer, harder to find, and less fair.
2 - I would say the other issue is having control, to an extent, of where you get loot. If players have to jump through hoops just to be able to kill some random enemy they may never encounter, or encounter very rarely that in and of itself is a problem. You could easily go through the game and never encounter these enemies, similar to the mythic goblin now.
Now you’re stuck holding onto a niche aspect or gear piece, just in case you stumble across a rare encounter that may never even appear.
I think a better design would be if these things were foreshadowed so to speak through events, or occurrences in the game. Maybe a part of the open world grows darker, or a world message goes out, ‘something dangerous has emerged in the land’ along those lines, so you know it’s going to have a better chance to spawn in specific areas. Maybe even a new affix to a NMD dungeon.
Random encounters shouldn’t punish players for not being perfectly prepared, especially when the encounter itself is unpredictable by nature. The butcher already does that in dungeons, which is great, but now you’re tying extra goodies to a random encounter that you may actually have no way of defeating, unlike the butcher, where you typically always have a chance, albeit a smaller or bigger one depending on where you are in your leveling/gear.