Ie.. don't make it a paid service.
As the game almost directly intends you to only play one character (via subscription tiers and class structure) id say its almost needed to provide fantasia either cheaply in game or very regularly via sub rewards. For me, its all fine and good while i've got alot of story and classes left, but once all that's done, I can see myself wanting a change, realizing i'll have to pay additional for it, and just decide its easier to go off and play one of the many mmo's i've already played. Start all over to get 2 fantasias? Hell no.
More positively, knowing i'll have the flexibility to change my appearance if i want to would help keep that same old character fresh. In other games you have the variety of different characters by their design (new characters for each class).. so this isn't a problem.
My suggestion would be to give everyone a complimentary fantasia every 1-3 months. It would be a positive for me to keep subbing at least.