Each level up, a planeswalker unlocks something new. It's often 4hp, additional mana per color match, or a leveled up ability.
Here's what I have so far, tested only with Chandra:
- 2: 1st ability is now lvl 1
- 6: 2nd ability is now lvl 2
- 14: 2nd ability is now lvl 1
- 26: 3rd ability is now lvl 1
- 31: +4hp
- 32: 3rd ability is now lvl 2
- 33: Your planeswalker regeneration improved
- 34: Your planeswalker regeneration improved
- 35: Your planeswalker regeneration improved
- 36: +4hp
- 37: Your planeswalker regeneration improved
Is there a comprehensive list somewhere?