Not a bad idea in a void. But it wouldn't work for XIV. Not without MASSIVE overhauls. Take BLM for example. Has 3 elements that each serve a specific purpose. Lightning is a DOT, Ice regens MP, Fire is your big burst damage. If they added elemental weaknesses and there was an enemy weak to ice, what would you do? Just ignore your rotation to spam ice spells? That wouldn't really add any complexity, in fact it would remove it since a large part of BLM's skill comes from getting all your fire spells in without losing enochian.
That doesn't even get into how none of the noncasters currently have any elemental abilities that I can think off off the top of my head aside from Ninja and maybe monk if you squint. They'd have to add all sorts of skills to them. And then what, does every class get every element? Or do only certain classes get certain elements? That would start making them preferred over other classes for specific pieces of content which is explicitly against the dev team's design ideology. They want everyone to be able to just play the class they want to play, to experience the fantasy they want to experience in that role. If only certain classes got certain elements, you'd be pressured to play different classes to be able to compete, so if you just wanted to play X class and that's it, you'd be SOL.
Once again, that's not inherently a bad thing (in fact it could be pretty cool), but it goes against this game's design ideology. It just doesn't fit with how the game currently is.
