Do you want a d20-based system, Hugo?
Characteristics:
To define physical and mental capabilities of a character, I suggest the S.P.E.C.I.A.L: Strength, Perception, Endurance, Charisma, Agility, Luck.
Those may very from 1 to 20, except luck, which can only reach 7. Exact value of characteristic or less on d20 is a success.
Luck might be rolled after a failed action. If succeeded, action is a minor success.
Now for bending - I suggest to add extra parameter for benders, which is bending skill. To define damage dealt by bending, bending skill is multiplied by 1/5 of parameter (rounded down) as listed:
Firebending - Strength OR Agility
Waterbending - Agility OR Endurance
Earthbending - Endurance OR Strength
Airbending - Perception OR Agility
Metalbending and lavabending - Perception OR Endurance
Lightning - Perception OR Strength
Bloodbending - Charisma OR Agility
For non-fighting bending activities, regular check is performed with modifiers applied at GM's will.
Benders, therefore, are rather balanced
Fighting:
If you want it heavy, try this:
Arms - 15pts+15pts - minus 2 to Strength and Agility if depleted
Legs - 15pts+15pts - minus 2 to Strength and Agility if depleted
Torso - 25pts - minus 4 to Endurance tests if depleted
Head - 15pts - harder to hit - all attacks directed at opponents' head suffer -3 penalty; if depleted, character gets black-out; If
Total basic - 100pts. Each point of endurance over 10 adds 2 points everywhere.
Various cold weapons deal damage proportional to d20 roll:
Sword - 1d20
Lance - 1d20
Axe - 1d20
Dagger - 1/3 d20 (rounded down)
Arbalest - 3/4 d20 (rounded up)
Arrow - 1/2 d20 (rounded up)
Et cetera, you got it. Impovise with exact valuse for weapons you encouter.
Also, to define the fighting order, perception may go as initiative. Number of action points, which define what can you do in your turn of fight - you Perception value.
Generic attack - 10 points, others may vary depending on verbal description.
If you have something else in mind, sorry for disturbance.