I have to admit I really have no idea how elemental resistances or even simple magical resistance works, but I think this suggestion is a bad idea. It would just make gearing more tedious than it already is. For example, I'm guessing if I'm K/M/W I will have to make two sets of accessories/gear for K/M (light) and M/W (fire) to maximize dps. This seems unnecessarily complex, so I would just prefer to leave it the way it is now, all based on straight magic attack/defense/resistance.
Don't you have 2 sets of gear now since knights wear armor and mages wear cloth? It wouldn't be anything different except a different type of gear. But that's also why I said instead of gear it could be a buff or runes that can be added to gear so that people didn't have to change existing gear and also have the option to utilize the function.
@Cike, my suggestion isn't only for classes that use magic or projectiles. Knights could have holy/light based gear, warriors fire, earth or whatever elemental gear would work best, etc. I'm just throwing something out there to make use of what is already in the game to make it more interesting (aside from fixing things). Plus, it'd be fun to hit someone with a flame sword or fireball and watch them burn for a few seconds.