It seems like there is always a push to create new classes to develop a character for every individual concept.
I think this is problematic. A character class should not represent a single conceptualized character. Otherwise, it would require thousands of classes to fit every conceivable idea.
Character class should represent a mechanic. A mechanism for developing the concept that you envision.
So you want to create an assassin class. Is it necessary? Can you create your killer using the rogue mechanic, or the wizard mechanic, or the fighter mechanic, or the cleric mechanic? If not, what is missing? Is there just a new power that you want? If that is the case, the solution is to create a new power, not create a new class.
This same holds true for other classes. We do not need 2 vancian spellcasting classes. We do not need 2 classes with martial damage dice and maneuvers. We do not need two classes with skill tricks. One class should represent each mechanic.
If you want new powers, create new powers but use an existing class.