One problem is that every class currently is an already existing class from a past Final Fantasy game. There are plenty of classes missing from the roster currently, but none that match those descriptions.

Maybe Geomancer for the elemental bit, but that would be location based and essentially broken, let alone hard to play. Or Blue Mage, but they would only use the abilities of Primals, not transform into them.

Don't be discouraged though, suggestions are always a good thing, they sprout further ideas. I recommend looking into older, not currently in classes from past franchises and finding ways to make them fit. Like Samurai, Pirate, or Red Mage.