Archnemesis modifiers should be tied to monster type, and consequently to map type
Ever since Archnemesis was made core, player agency in responding to the challenges it brings has dropped dramatically. In most cases you either kill a monster comfortably or run away if it refuses to die, or just die yourself if it oneshots you. It's not a very interactive system, to say the least.
One way GGG could improve player agency with Archnemesis modifiers is by making different modifier types much more likely to appear on certain monster types. It makes much more sense to see animals with frenzy, spiders with toxic, undead mobs with soul conduit, etc.
This would also mean that, outside of maps with overridden mob spawns (area is inhabited by X), you can roughly predict what kind of archnemesis mods would spawn in a map and how comfortable you would be. A CI character would, for example, feel more confident in a toxic sewer map than someone with -60% chaos res. Doesn't that make way more sense from a gameplay perspective?
Of course, it would only be 'more likely' and not 100% predictable. Mobs with random modifiers would still be possible, but this way their randomness would appear as variety from the norm, instead of as the norm.
edit: tying it to monster type would actually make 'inhabited by X' mods also point to more predictable outcomes, like more monsters with kitava-touched in areas filled with his followers.