I haven't been able to find much information regarding exactly what the 'Fantasia' potion does. I've heard there's two separate versions, one given to players who had 1.0 characters and another as a reward for the end of the first month of a subscription. I have my eyes set on the latter, though some people have said you can't change your character's name when using it which makes me a tad wary! If you can change every aspect of your character beyond their classes, however, I'd definitely consider using it.
