Well, I know that you're currently working on the six 'core classes' atm, but I'm going to throw in my 2 gil, which will primarily concern the more... unusual classes...
FF is FF because of a wide variety of things. Chocobos, summoners with horns... as FF players, we all understand what these are.
Chocobos are relatively easy. I worked out a statblock for yellow chocobos in around 15 minutes.
That brings me to my next point... summoners. Using summoning magic, regardless of which particular system is used, will be rather difficult. Admittedly, something as simple as using a variant of summon monster spells would be alright, but I don't know if that quite fits. Not only that, most of the 'common' summoned monsters aren't run-of-the-mill critters... some are, such as the chocobo summon from FFIV, but others, such as Shiva, are a little more unusual and will require more work.
And the blue mage, which is also based off of monsters. In the past, I have attempted to create a blue mage-like class for DnD. It doesn't work. Monster abilities are generally either too wierd to use for blue magic, or emulate spells. While blue magic does generally end up emulating normal magic, in some cases, it doesn't... imo, a blue mage class in FFT would've been an excellent idea, as there are a wide variety of monster abilities that other magic types simply lack.
There are a few other classes that will be difficult... but I don't think that most will be as difficult as these two will be.
Oh, and if you happen to need someone to stat up either of those classes, or anything else, for that matter... well, I am available...