How can they add more classes and make them unique and have their own flavor?

[Ultimatecalibur]

Well DRK begins very similar to GLD and AST is essentially WHM until 50, or was it 40. I've not tried MCH yet.

Dark Knight may start out similar to Gladiator, but it ends up in a very different place once it gets Dark Arts. Much of its high level play is built around proper management of MP consumption/generation.
Much of the Astrologian's play is built around proper use of its cards.
Machinist uses a fairly unique proc based combo system.