I'd like to see them in the game again. Things like the older golems, where most damage spells did horribly piddly damage, if any and maybe one or two that did decent damage, but usually still reduced. I'd also like to see more like that. Not just monsters that have an immunity to a single element or something like that, but actually reduces magical damage suffered. They could be designed with somewhat lower health and AC, making it more ideal to beat on them.
I'm not suggesting they be scattered throughout every dungeon, but encountered frequently enough to keep casters on their toes, much like how traps pop up fairly frequently. Some dungeons may even be more themed with magically resistant foes, making it more of a challenge for casters to go through. I personally think this would help bring some balance to the game, as well as offer it more variety and challenge.