Would be nice for assassins with lots of useless herbs :D
Also, this would be especially thematic for FGJC.

ha didn't think about the alchemy implications (having said that, alchemy when you are fire immune + ring of ice is incredibly OP)

yeah maybe the FGJC greater vault should always be a greater ice vault. Dunno. Not sure it makes sense thematically to get a fire vault in those caves.

A bit skeptical about frost salamanders in huge quantities -- that's basically guaranteed cold immunity. Maybe ice lizards should be enough?

It's not like players by this point don't already have =cold from AMW.
True, intrinsic immunity is always better but it's not really such a huge advantage.
Without it, you simply have to switch armors for different locations - powerful fire and frost monsters together appear very rarely to require the PC to have both immunities to survive so I don't see much of a problem here.
Also, there's plenty of those lizards spawning around FGJC and IQD already to warrant lots of corpses anyway.