I think a couple of the recent interviews mentioned this, and now we have it in the new Live Letter. I really love this idea, and it makes since because previous FF titles have sort of mixed and matched the roles of Thief and Ninja. I know its probably a long ways off (priority on more mage classes and pet classes first), but I cant wait for some THF/NIN goodness. And no I dont want them to be a tank class.
I would really like it to be fast attacking stealth DPS firstly. However, it definately should play a role in helping out with things like opening locked doors in dungeons or opening locked chests and what not. A steal ability would be nice as well, and though not necessary, I wouldnt mind it having a Treasure Hunter effect either. Then by making the Job for thief be Ninja they could introduce a broader aspect to the class. Like maybe it could add on to stealth DPS with enfeebling/crowd control measures (sleep/stun/bind/etc. and slow/paralyze/poison/etc).
Whatever the case I am all over this. Class = THF, Job = NIN, yes please.