The Ossiranth Chronicles

I've been working on this for a little bit, but it seems I've been having problems with class advancement. At the moment, I'm working with a model where players learn any abilities they want. They can undergo certain quests which test their skills with certain class abilities so they can advance their class levels. For example, if you pass the Mystic test, you become a level 2 Mystic.

After a certain number of tests have been taken, then things begin to branch out. If you have so many levels in a certain class, you can take a test to unlock the second tier of that class. The Mystic, which is the basic spiritual class, would unlock the Kinetic class and can then begin learning Kinetic abilities. If you have so many levels in a second class, Communer for example, you can take a different test to unlock the Shaman class and begin learning Shaman spells.

The problem I seem to be having is a balance on class diversity. Some classes have more combinations than others. The Squire class has 4 different combinations at the moment, while the Brute class only has two. It just seems like it wouldn't feel right if a player started heading down the path of the Brute and could only become a Barbarian or a Berserker through class combinations, while the Mystic can become a Shaman, Martialist, or a Sage. Certain classes only have limited options while other have many more.

It's not such a huge issue since players can go and learn other classes at any time they wish, but the Mystic could conceivably combine with almost any class while the Rogue couldn't. And don't get me started on third tier ideas. Just thinking about potential imbalance for that tier makes my face hurt.

Is it better to try and force combinations for the sake of balance, or would it be better to just forget about some combinations so that it feels like an even amount of choices?