Thief is a big hole in the existing job lineup.
One of the more obscure FF jobs might be cool to see, time mage maybe, or oracle. There's a lot they could do, but they're limited by the armory system. They'd almost have to add new weapon types for any new job they roll out.. either that or pull some shenanigans where a job has a multiclass pre-requisite, and being the job enables the use of different weapon types than the classes that got it.. for instance something like:
- Equips katanas as Ninja only
So what you'd have there is a weapon type that isn't equippable by any class, but can be used by Ninjas while they're Ninjas.
It's pretty wacky, but it IS a way around the armory system's limitations.
If random outcomes aren't acceptable to you, then don't play with random people.