Since you’ve have shown that the system can merge abilities I have an idea for a long term reward.
When Levi max out of the main four bending styles wait on combining them into one skill, let him also max out metal, lightning, wood, and energy(yin/yang) so they combine for a secret quest trigger to unlock law bending based on the Taoism or Shintoism concept( not to familiar with it(definitely didn’t just think of it)
Just food for thought