I would create a shapeshifter. Because WoW is so highly referenced when it comes to shapeshifters, I guess I'll say that the shapeshifter I'd make would be akin to the "druid". Only, rather than animal forms, I'd want the job to take on unique monster forms. I don't mean monsters that we have to hunt down and kill ala blue mage, although that could be an aspect of this job. I mean monsters from millennia past, essences which have been long kept by the circle of practitioners whose generations safeguard them. However, because FFXIV is set up differently from another MMO we know so well, there'd be no way to use the monster forms to be like, "Okay, I'm going to tank this time," or "Okay, I'm going to heal this time." So I'd have to make this job either a DPS or a Tank.
One thing I know I would enjoy would be if the monster forms weren't part of some kind of combo sequence. Changing forms changes skills and abilities. One form might play like a caster, one form might play like a melee. Maybe I'd create a high level ability which would allow the player to "form dance" freely, allowing respective forms' skills to chain together to create a unique combo, but form shifting wouldn't be required.
I'd call it the Morpher.
EDIT: Because I know a lot of people in the FFXIV community love aesthetics, the Morpher would have a fun little ability that allows them to briefly copy the appearance of another player. For maybe an hour. Getting into combat would break the transformation. The Morpher's main utility is transforming, so it wouldn't make sense to be able to hold onto the other players' forms anyway.