Look, you can either have a limited number of in depth classes with balance or a ton of more generic classes with balance. You cannot have both, WoW has taught us that, the work load becomes insane after awhile.

The more classes and abilities you add the more you have to account for. 1 class is not just 1 more thing to worry about, a new class comes with a ton of abilities you'll have to worry about and how they interact with everything from other jobs to the enemies in the game. That's just PvE, it doesn't even include PvP balancing. The more unique you make it, the harder it will be to tune. The more time they spend tuning class balance the less content you'll get so they make the classes easier to tune thus more generic.

If you want more depth to your classes and more strategy behind the game play then it's best to start asking they slow down on new class releases. Otherwise, it'll become more generic with each subsequent class they put out.