the trouble about the thief as jobs is the lack of new capacity that will not look like a thing ninja will use....
outside of the steal capacity, thief haven't generally a lot of stuff.
bravely default one of the most recent game using the jobs system from SE, the thief as action have 5 skill... all the rest is passive.
- steal, you obviously steal an item
- life steal, you steal hp
- mug, you steal an item while attacking (25%)
- Godspeed strike, you attack really fast (like a ninja) an enemy
- shake down, steal item at every enemy (25%)
indeed bravely default is not called final fantasy but still is part of it and we can argue that it's a solo game. but you can check every Final Fantasy before, that about what the thief get each time... the class is here for age, but it's more because it's something commons... do it deserve it own jobs in FF14 ARR? no, clearly no. since we can't steal stuff nor gil, the only stealing left are HP (already used by the marauder and dragoon) or steal stats (it will lead to some trouble, but still possible)
it's why i think that the thief have no place as jobs... it will have no mechanic really interesting or unique.... and finally it don't enter in the lore of the jobs crystal that something extremely rare. (ok not that rare with the number of player hahaha)
ps: we can argue too that warrior is something commons, but it's more a mix between a berserker and a viking more than a warrior.