I like when people come up with new possible classes that Square Enix may likely have never thought of adding to the game, or may have added in the past. To me, it's all fan fiction, and I love fan fiction. A while back I tried to write a fanfic that never really got off the ground, about what I thought "Final Fantasy 15" (it wasn't announced back then) would have been like. The main character was a Rune Mage (Silver Mage/SIM), whose abilities centralized on the metamorphosis of the world around him, including various metals from which weapons are made.

A lot of it appeared an awful lot like telekinesis; whenever he fought, he wielded swords/daggers/hammers/axes with his mind, and he could shapeshift weapons or meld two/three into one.

I think it'd be cool if they added something kind of like the "Rune Mage" I described earlier. I think it'd be most similar to Red Mage, from the stories I've heard.