That was why I said the weaponmaster path is available to benders.  He spent 10 on getting dual swords 4 and and enhanced that with his bending.  He's taken some stunts from both.

Next step.  Non-Bending stunts.

These stunts are available to all characters, nonbenders or otherwise.

The first level costs the same as snapping(10)
Weapons Mastery.  Cost 10.  Gain -Name of weapon here- at rank 4.(I'm debating how to balance this with higher physique scores...  Perhaps have it work like Pewter...  I think that might be a little OP, given the lack of a cost in metals and the like.


Stunts
Weapon Damage +1
Weapon Defense +1 Defense Dice  Can be taken twice.
Polearm Vault-Can use polearms to gain +1 movement and +1 on physique rolls to jump.
Multishot.  Multitarget stunt as seen in steel for ranged weapons.
Armor piercing attacks. Attacks ignore 2 damage reduction.
Chi Blocking-(Fist weapons only)  Any attack that causes a burden removes the burdens value from the affected characters bending rolls.  All burdens should disable limbs.
Greater Chi Blocking-The negatives to bending rolls are doubled.
Greater Weapon Defense.  The character may forgoe Weapon Defenses extra defense dice for a single reaction roll per round using their weapon rating.
Long Range-Increase range of ranged weapon attacks.

I would say try it out but don't be afraid to tweak or abandon it should things not work.  Everything needs balance tests.  I'd be careful though. 
As for the trait idea.  If they start breaking their old ties have it become an attack on their standings.  People start to not trust them, eventually they get the trait disloyal and a new enemy. It should work.  The first rule of homebrew is only homebrew when what you want cannot be done within the RAW. 

Thirded and Fourthed.  I really want to see this.

The system is designed to be far more party run than usual for an RPG.  In D+D the DM throws a dungeon at the players and they cut their way through.  In Mistborn the characters make a scheme and run it based on their goals and abilities.

As a DM I find starting things like that hard.  
My formula, which might help you get started is this.
1: Set up an antagonist.  Have the villain attack or a friend get murdered or a noble kidnap a pretty girl one of the characters likes.
2:  The party arts up and executes their plan.
3:  Big confrontation
A: Make sure to hide hints and things that will further the larger story within your individual sessions.
(This system was created and honed for The Dresden files RPG.  It doesn't work as well under the final empire, but with a little work it translates well.  You just need to undercut the scope of
 Mistborn a little.  Set the large plots later and have the inquisitors less ready to attack early on.

For instance my current worldbuilding is for a CSI style campaign in TLR's less watched vacation home on the other side of the planet.  A place where Slash have more freedom and nobles less.  On the other hand the gang of Hemalugist cultists living in the abondoned mines and caverns beneath the city are causing quite the stir.

I think the largest problem I've had was motivating a Thieving crew without a good way to accrue resources.  An Obligator crew hunting down gangs and murderers in a city designed after Gotham though.  It's going to be awesome.

I would think the easiest step would be adapting warbreaker.  Wait utility Stormlight 2 to really work on WoK.  Too little has been show to do more than just groundwork.  Warbreaker is pretty demystifies.  Elantris could get started as well.

Due to lack of disagreement I'm moving forwards.

Quick changes.  Making water have a 100 lb weight limit.  That's 12 gallons of water which seems better than 6.

Airbenders also can move vertically with their movement but not levitate.

Earthbenders have range 2 not 3.

All Benders have reaction rolls against elements they can control.
Stunts

Water
Enhanced lifting. +150 lbs.. Can be taken multiple times.(note that if there is something on that then the weight is part of this limit.  250 lbs allows a waterbender to psuedo fly.  

Enhanced Range. Gives +1 range.

Compression.  Add +1 damage to all waterbending attacks.

Water armor. Grants 2 defense dice while bending.  Requires at least 20 lbs of water.

Plant bending
Allows North and south water tribe Benders to manipulate water within plants.  Weight limits apply.

Ice bending.
Allows Foggy swamp waterbenders to freeze water into ice.  Up to three cubic feet.

Enhanced freezing.  Can freeze up to six cubic feet.

Enhanced swimming.  Add 1 to swim speed.
Multiple targets
As seen in Steel.

Healing
Allows a waterbender to remove 1 physical damage.  A waterbender may spend a short breather to heal a serious burden.

Advanced healing.

Allows a waterbender to remove 2 physical damage.  A waterbender may spend a short breather to move a grave burden to serious.  They may spend a long breather to heal a Grave burden completely.

Master Healer

Allows a waterbender to heal 3 physical damage.  A waterbender may spend a long breather to move a mortal burden to a grave burden.

Of note.  No injury(result of 1 attack) may be healed more than once.  No burden may be healed more than once.

Blood bending- prereqs- Waterbending level 7.  During the full moon a Blood bender may control water within a human body forcing that person into action.  All actions taken under the influence of bloodbending are done at -2 dice from the targets initial state.

Master Bloodbending-  prereqs- Waterbending level 9.  Bloodbending no longer requires the full moon.

Bloodbending is resisted by willpower or waterbending, whichever is higher.


Earth stunts.

Enhanced lifting
See above

Enhanced range
See above

Hard Rocks
+1 damage

Multiple targets
As seen in Steel.

Earth armor
Armor 1 while bending.  Requires at least twenty pounds of rock.  Can be taken multiple times.

Plow Sprint
+1 move speed.  Can be taken up to twice.

Tremor Sense
The character can perceive everything for fifty feet around them that is touching earth.  This ties to wits for purposes of perception tests.

Tunneling
The character may move beneath the earth as they would above it.

Metal Bending
Prereq-Earth at level 7
The character may bend metal as they would earth.  The earth armor stunt provides an extra +1 armor when the armor is made of metal.

Fire Stunts

Enhanced range.
See above

Blue Flames
+1 damage.

Multiple Targets
As seen in steel.

Fire Shield
+2 defense dice.

Explosion
Your bending attacks can strike everything within 1 range increment of the target.(pairs well with multiple targets for huge armies but might just be multiple targets...)

Fire Jets
Allows sustained flight via firebending.  Follows the same rules (roughly) as Steel pushing.

Flamewalk
+1 speed.

Crippling burns.
When your attacks injure an opponent they count as 1 burden level higher than they normally would.

Lightning Bending
Requires firebending level 7
All firebending attacks do +1 damage and gain a free nudge.

Lightning redirection.
When hit by a lightning attack the firebender may make a wits roll against the opponents Firebending to send the attack at a different target with their own firebending roll.
This is a reaction roll.

Air

Enhanced Range. 
See above.

Air Bullets
+1 damage

Wind Shield
+2 defense dice.

Air Scooter
+2 movement

Wind lift
An airbender may move objects of any type around Up to 100 lbs.  This stunt may be taken up to 3 times.

Glider Speed.
An airbender may fly up to 75 miles per hour with a glider instead of 35.  They get an additional +2 to dodge while gliding instead of the original + 1.( Note that gliders are a prop worth 2 slots)

Channel the wind.
Airbenders gain a +1 Circumstance dice on all airbending rolls in windy conditions.


 

The damage things are just weapon values for the element.  For instance-A coin is weapon +1.  A Bag of coins is Weapon +2.  I felt that earth and fire should do the most damage.  Most extra things done with bending,-Earth armor, air scooter, etc should be stunts.  This is just the base level to keep the bending from overpowering mistings.(IE I'm trying to keep things in balance with the game as is.)

Any thoughts on the base mechanics I posted about a page ago.  ONce that's done we can start on stunts for each and work our way through into a full system.  Healing will be one of those stunts, if not half a dozen. 

I agree that for now we stick to the source material as best possible.  Anything to say on my base bending stats.

See my previous post for stuff.  And where did that info come from?

Do we know Bumi doesn't bend?  I'm still theorizing mom's waterbending.

And we should wait on Korra until we get the base system set up, as updating to Korra-tech will require some work.

Basic Earth bending
Lift to a certain weight-100 lb's of earth.
Throw up to three range? +2 damage.

Basic Waterbending
Turn up to three cubic feet of water to ice.
Throw up to two range- +2 Damage
Lift up to 50 lbs of water.
Swim Speed of up to three range.

Basic Airbending
Increase move range by +1
Throw up to 3 range +1 damage
+1 defense dice

Basic Fire-Bending
Create Fire
3 range +2 damage

True, but what we've been discussing is largely used by Tai-li and the equalists.  Spirit bending should be left to someone who can bend all the elements.(As having rogue spirit benders running around could go very bad very quick(Could be a fun story arc, but we haven't seen Spirit bending do anything but take away bending.)

The third one is spirit bending, not chi-blocking.

I would like to point out that from a game perspective letting the avatar position not be exclusive is a good idea.  But that's more a DM choice thing.