The upcoming Class Mastery passives are a perfect first step to balance subclassing.
I'd like to suggest a second step: a simple Class passive rework. Do a full pass on all Class skill lines and tweak them following a pattern where each Class skill line has:
- 2 unconditional passives
- 1 passive that requires slotting a skill from that line on the current bar
- 1 passive that stacks based on the number of skills slotted on both bars from the base class of the skill line
That approach would make the fourth passive significantly stronger for Pure Classes, and offer a simple universal control hook for devs to tune the gap. It also means that devs could shift back to the far superior design philosophy of "one-main-role per class skill line" seen in DLC classes.
They could also make things that enhance "Class Skills" (such as the Class Embodiment bonus in IA or the unique 5% Solar Barrage buff) only apply to the base class (wherever that's not already the case).