Mostly I just wanted to have a tail. :)
If you can explain WHY the tail is there, go for it xD
The only thing that comes to my mind is someone wanted a pet animal-girl or something, but I'm tired right now
I was hoping a Mystic Knight would be a front line fighter himself, but with a magic sword skill similar to Steiner of FFIX. But if the class is more magic focused or too light to be on the front line as a tank then I'd probably pick a fighter, too. Maybe if the black mage can enchant swords that'd be great for a 'dual tech' sort of approach.
Mystic Knights are based on their role in FFV, they're frontline fighters, more than spellcasters. They're not as strong as warriors, but make up for it by being able to enchant their weapons with elements or status effects to make themselves very versatile.