If you could pick one class to add now, what would it be?

[Moth]

A unique pet class like Puppetmaster, but that isn't happening anytime soon so I would go for ANY new mage class.

People ask for samurai, ninja, blademasters and such but we still have only two mage classes with basically the exact same gameplay.

Well Arcanist seems like a mage-type class and it should be interesting o:
They already have a guild in Limsa too so I am sure it will be added in the near future!