We're getting Gunner and Time Mage, and we HAVE Thief.
They're called Machinist and Astrologian, and it's called Rogue/Ninja.
Yoshida & Co. already gave their rationale for why Rogue is just a different name for Thief. The fact that ROG/NIN have all of the "iconic" thief skills really should have been tip off about this. Trick Attack, Sneak/Hide, Mug, Sneak Attack...all normally thief abilities. There's no use in adding a class that would be redundant on conception.