I bring this up here because I'd like to hear other classes' opinions on it before bothering an Envoy.

Would an additional defensive spell in each Elementalism school severely imbalance the game? I'm thinking it'd be learned up in the vicinity of Staff (which is, uh, up there somewhere...). Aside with general protection, each would be designed to protect against the opposing element (Aquamancers would be able to resist a little fire due to a handy-dandy layer of magical ice, Geomancers would be able to ground themselves properly to take less damage from lightning , etc.). This allows for increased defensive capabilities on the Mage end, but also skews it to have the more interesting aspects of the defense directed towards resisting other Mage types.

Good idea? Bad idea?

The way I see Knights would be very defensive, guardians in the middle, and Magi as very offensive, like the Monks of Lusternia.

I always thought of it the other way around. Knights very offensive, since they're there right in the thick of things smashing people, guardians less so since they at least partially rely on the aid of others, and mages most defensive, since they're supposed to be all holed up in their demesnes slinging runes at people and so forth.

That's probably a pretty good idea since mages have pretty low defence now...I think it might help to get the "opposing elements" sorted out differently though. In Lusternia, it's more about Fire Vs. Air and Water Vs. Earth. So the Water dudes would have like a poison dilution barrier, and the Air dudes would have a windwall that diverts fire around them. Fire could have a sort of fire cup that gets fueled by wind blasts (I don't know if they're going down the lightning route, though) and Earth could get a very thin, porous rock wall that split a steam blast into several vents so the damage is less localised and therefore less concentrated.