Brick Bat:
Adding a new Brick Bat Shield move, when pressing Shilt + Use the ability goes on "Shield Mode" basically exactly like Mera Mera no Mi Shield ability (Kyokaen) and Beta Beta no Mi ability (name too long), by pressing shift + use again the ability go back to attack mode.
(Brick Bat Shield ability have been seen in episode 358 in the anime and in chapter 463 in the manga)
Doppelman:
Doppelman is litteraly an immortal clone of the user in One Piece, I suggest to make the Doppelman have the same hp as the user for exemple if user is max doriki and max health is 100hp then the Doppelman will have 100hp and if the max health is 1000hp then the doppelman should have 1000hp too, adding this I suggest to add a little passive regen to the doppelman and more resistance, like this the doppelman will be able to tank hit and regen (like it is an immortal being) but will be destroyed if hit too much time by strong ability.
Since Doppelman is suppose to be a clone of the user it should grow stronger in the same time as the user get stronger, I will suggest to buff damage of the base Doppelman, and every time the user hit the lvl 25 in his buso haki xp (lvl 25/100 - 50/100 - 75/100 - 100/100) the doppelman will get stronger and stronger, also since the Doppelman is the clone of the user + it is an ability, when the user use haki to make his attack stronger and hit logia user, the Doppelman will also get the buff from the user haki (be able to hit logia user mostly).
Nightmare Soldiers:
I will suggest to slightly buff the health and resistance of them, slightly buff the damage too, but they get ONE SHOT by all fishman race ability (since salt cure the zombies).
Shadow's Asgard and Shadow (the item):
(This one is a totally rework ability)
-New Passive (Shadow Absorption)
This new ability will show a shadow meter next to the combat bar (just like Magnetic Item passive from Jiki that show number of iron item you have or Karu's karma meter)
This will show how many shadow the user have in his body. (the meter won't grow when you eat shadow)
-Shadow Item:
Now by stacking 10 shadows in a crafting table you can craft the new item "Shadows x10", consumming this will give resistance 2 and strengh 3 for 1min30 but unlike normal shadow it won't stack when you eat more of it, if you eat more of it it will only reset the timer.
If you stack 10 "Shadows x10" you will be able to craft the new "Shadows x100", consumming this will give resistance 2 and strengh 5 for 1min 45.
-Shadow's Asgard:
The new shadow asgard once activate will have a duration of 1min45, it will automaticly consumme shadow from your inventory (shadow, shadows x10 and shadows x100 item will all be consummed) the ability will consume 100 shadows every second, every time a shadow is consummed the shadow meter will increase up to 1000 once the shadow meter is at 1000 the ability will stop consumming shadow.
Once you activate Shadow's Asgard eating any Shadow item won't give any buff.
When your Body activate Shadow's Asgard every time you pass a certain lvl of shadow in the shadow meter, shadow asgard will give different buff that will all stack together and last for the whole duration of Shadow's Asgard.
-Shadow Meter at 10:
You and your Doppelman will get a little bit big
You will gain +1 attack damage modifier
You will gain +1 toughness bonus
-Shadow Meter at 100:
You and your Doppelman will grow even bigger
You will gain +2 attack damage modifier
You will gain +0.5 toughness bonus
-Shadow Meter at 200:
You and your Doppelman will grow even bigger
You will gain +2 attack damage modifier
You will gain +0.5 toughness bonus
Brick Bat will have +1s of ability duration (normal mode will launch more bat since longer time and in shield mode will shield even more)
-Shadow Meter at 300:
You and your Doppelman will grow even bigger
You will gain +2.5 attack damage modifier
You will gain +1 Toughness bonus
Tsuno Tokage hitbox and size and will be increase by 10% and damage will increase by 5%
-Shadow Meter at 400:
You and your Doppelman will grow even bigger
You will gain +2.5 attack damageModifier
You will gain +1 toughness bonus
-Shadow Meter at 500:
You and your Doppelman will grow even bigger
You will gain +2.5 attack damage Modifier
You will gain +1 toughness modifier
You will gain the slowness 1 effect
You get Knockback resistance
Black Box will now shoot 3 projectiles instead of 1.
-Shadow Meter at 600:
You and your Doppelman will grow even bigger
You will gain + 2.5 attack damage Modifier
You will gain the resistance 1 effect
Brick Bat will have +0.5s of ability duration (normal mode will launch more bat since longer time and in shield mode will shield even more)
-Shadow Meter at 700:
You and your Doppelman will grow even bigger
You will gain the strenght 2 effect
You will gain +1 toughness bonus
Tsuno Tokage hitbox and size will be increase by 15% and damage will increase by 10%
-Shadow Meter at 800:
You and your Doppelman will grow even bigger
You will gain the strenght 3 effect
You will gain +1 toughness bonus
Brick Bat cooldown is shorter by 20%
-Shadow Meter at 900:
You and your Doppelman will grow even bigger
You will gain the strenght 4 effect
You will gain +1 toughness bonus
You will gain the slowness 2 effect
Tsuno Tokage cooldown is reduced by 20%
-Shadow Meter at 1000:
You and your Doppelman will grow even bigger
You will gain the strenght 5 effect
You will gain the resistance 2 effect
Black Box Cooldown is reduced by 20%
Brick bat and Black Box damage is increase by 10%
When Shadow's Asgard ability is turn off / goes on cooldown (the cooldown is the same as the time you use it + 20s)
The user will loose all effect written above and suffer from the following debuff:
Slowness 1 for 45s
Hunger 3 for 45s
Weakness 10 for 45s
Dizzy effect for 10s
Nausea effect for 20s
Blindness effect for 45s
So in total when using the new Shadow's Asgard at full potential (with 1000 shadows wich will only be posible with the Shadows x100 item and it will be very very hard to get 1000 shadows so the transformation will be very strong but hard to get) you will gain the following effect:
You and your Doppelman massive size (and so reach too) increase (same height as Mega Mega would be the best).
You will gain Resistance 3 effect
You will gain Strenght 5 effect
You will gain 8 Toughness point
You will gain 15 Attack Damage
Brick Bat, Black Box and Tsuno Tokage cooldown reduced by 20%
Black Box shoot 3 projectiles instead of 1
Brick bat last +1.5s more (meaning shooting more bats in ATTACK MODE and the shield lasting longer in SHIELD MODE)
Tsuno Tokage HitBox and size will increase by 25% and damage increase by 15%
Brick Bat and Black Box damage will increase by 10%
You will gain Slowness 2
My ideas behind all that was to improve Kage and make it way more versatille and more One Piece accurate, Brick bat will be defense and attack, Doppelman won't be instant one shot by other player will be more usefull and not just for the switch (Kagemusha) and will finally be able to damage people with haki and logia user, the Nightmare Soldiers will become a bit more usefull won't be one shot too except if you use salt based attack (fishman race abilities) and will be able to do a bit of damage on people now but will still won't be able to hit logia user since zombies can't use haki, and finally Kage low damage will be balanced with the new Shadow's Asgard that will insanely increase the damage, buff kage abilities and will give the user a very strong defense, while making the user and his Doppelman bigger (but the Doppelman will get only bigger and not stronger or more tanky because he just follow your shape so bigger but he is not getting buffed because he is not consumming shadows like you) overall this will make Shadow's Asgard very strong (wich is accurate Moria was half conscious and still able to maintain 1k shadows inside him, tank some of Luffy's strongest attack before timeskip and finally was able to cut Thriller Bark in half with 1 single punch (you can see it in episode 374 and in chapter 481 of the manga).