What Happened with Items

There is no way around it: Path of Exile needs very generic monster Modifiers that do exactly what they say they do. However, Archnemesis League is a success in so far as it showcased how fun fighting crazy strong rares can be. Basically, it was what Metamorph was hoping it could be.
The current system is close to getting it right: The more common modifiers are more generic, so that most of the time you hit 2 generic Mods like 'Gargantuan' + 'Steel-Infused'. However, since all Mods are lumped together, they feel more like "a Mod" instead of "that Mod". The Solution is make a clear distinction between generic Mods and non-generic Mods, likely with non-generic Mods having Loot Conversion.

Chaosweaver has resistance to Chaos AND Resistance to Wither.
Most Chaos builds play Occultist, which very likely utilizes Wither as a damage-scaling tool, and may even spec into the passive tree for increased effect of Wither. This nerfs the damage twice, and multiplicatively, in a single modifier while also adding chaos damage to the monsters attacks and spells.
The Power gain against Characters is substantially diverse, which leads to a mismatch between effort and reward - this is acceptable - but it occurs too regularly to be ignored, especially since Chaosweaver, for example, can roll on Magic Monsters aswell.
The scarcity of Mods is exacerbating this effect, of course.
A Likely Solution here is to split modifiers. Chaosweaver for example could be really good: Adds Chaos Damage and Resistance to the Mob. If you want resistance to Wither to be able to roll aswell, add a different Mod for it; Call it Withered, let it be immune to Wither, and give it a Chaos DoT Aura with low damage that is capable of inflicting Wither, scaling max stack application with cast speed. This would also greatly help with clarity; Chaosweaver is Chaos Hit, Withered is Chaos DoT when near. No need to learn a list of effects for each Mod :P
Another example is Droughtbringer, as it disables Flask-based builds almost entirely. While not as common as Chaosweaver, it is still occasionally seen.
The Design seems good in principle, interesting effect, nice challenge for many builds, to have to fight without flasks. But there exists an edge case where it disables an entire build on its own. This obviously, is a problem: Droughtbringer is not opt-in, but mandatory, since Archnemesis encompasses the entire game. There is no straightforward solution to this problem. This is not a double dip, but a simple modifier. While I believe many players have seen this as a very predictable issue, maybe the design team has not. Maps that have reflect Mods can be rerolled or vendor reciped. Rare Monsters, however, are involved in every aspect of the game except boss-fights. The only workable solution I can find with my capacities is to make a lot of Archnemesis Modifiers opt-in, as in players "pick their poison." This would also allow bringing back of effects like "Immune to Freeze, Chill and Brittle".
Slap Righteous Fire on a Mob, Give it Freeze Immune, and done.
There is another issue though, and that's player incentive. As it stands currently, Simply having such a mod in existence would help because of the scarce Modpool, but it would not really make it an important choice but rather a "Oh my build can't fight this mod" type of decision, which leads us to:

Because it is the worst culprit in my opinion, let's use Droughtbringer again as an example. The Conversion makes thematic sense (this is good), and the Mod is iconic enough for the reward change to be memorable (this is also very good) however, the reward changes into something dramatic that is likely utterly worthless, namely normal rarity no quality flasks, sometimes a unique flask worth 1c that no one on SC Trade will buy, ever.
Thankfully, the fix is to tune up the Conversion Mods, as either will likely work well. This is also a great opportunity to make Archnemesis mods interesting and, most importantly, worthwhile doing. Add exclusive and/or good/rare modifiers to the Flasks that drop, such as enchantments, unnaturally high quality or maybe even spacial base types. Also, don't forget that "Conversion" means turning something into something else. The Conversion should at least reimburse the opportunity cost of not dropping generic loot. That being said, the Sky is the limit, but it needs to be less generic; For reference, see 1.I.