This could affect pvp pretty badly. No one enjoys fighting when they’re at a disadvantage. What if leveling up unlocked stances that affected stats instead?
Attack Stance: +2 Basic Damage, -6 Max HP
Support Stance: Skills heal 2HP more, -3 Basic Damage
Guard Stance: +6 Max HP, receives 2HP less healing from all sources (minimum 1HP)
Any hero could use these without throwing the balancing off much. Technical ones might need to be restricted on certain characters:
Agility Stance: All >1 turn cooldowns -1 turn, -3 damage to all offensive skills
Dexterity Stance: Can Basic Attack twice while all skills are on cooldown, all cooldowns +1 turn
Deflection Stance: -1 damage from attacks over 7, +1 damage from attacks dealing under 7
The system would best work unlocking these 3 at a time, meaning heroes go up to level 3. With the amount of characters and rate of new ones, no one should have to grind to be competitive.