So im just thinking here, if there would be a possible way to balance out a game like this were you equipped skill lines to level, without class requirements.
It would function much like weapons and armors currently function. You equip the skill line as it would be more like a weapon or armor, and then as you level it levels, unlocking the skills/passives. Just add three slots for three skill lines.
My question would be more aimed to the min/maxers would would no doubt turn this game by force into a "these three or gtfo" how bad this would ruin whatever shred of balance the game currently has.
Also I wonder if this might make the addition of new skill lines a bit easier...since you wouldn't have to create a complete class worth, you could just add say an illusionist or conjurer line.