Please make THF it's own unique job, not relegate it to a class. Not that it would even make that much sense to suddenly go from daggers to katana. At the very least, do something like:

Class: Rogue

Job 1: Thief
Job 2: Ninja

Taking away from another post of mine, you could do this:

Give THF a large number of unique abilities.
Give NIN a large number of unique abilities.

Give Rogue a small number of unique abilities, that are NOT equippable when switching over to the job.

Doing this will give each class and job a lot more identity than if THF and NIN use all Rogue abilities, and only have a smattering of abilities to set them apart from each other. (Random example: If, at level 50, you have 5 unique Rogue abilities, you could shove in 10 unique abilities for THF and NIN rather than just 5 - not that you couldn't shove in more abilities if you wanted to, of course.)