thought id suggest a little game, coming up with a keyblade transformation for a keyblade from a previous game in the hypothetical scenario that it comes back as dlc 

just a couple to get us started, fenrir and lion's heart could easily transform into the signature weapons of the ff characters they represent, so fenrir becomes the buster sword allowing you to tear through enemies with like cross slash and what have you and lion's heart would become a gunblade with rapid attacks and an expolsion at the end of combos, their finisher would natuarlly be the most powerful moves of cloud and squall 

Fairy Harp: Turns into a dagger or cutlass like would be used in Neverland and allows Sora to temporarily fly freely. His attack speed is given a small haste effect to account for the smaller weapon and his movement speed is also give a small increase. At the end he does a flurry of attacks on the closest target in a small radius similar to the Peter Pan summon in KH2!

Perhaps Jungle King from KH1 could turn into a spear that could be used for extra melee reach, or even with a throwing attack.

Oblivion would need a vicious transformation, maybe a gothic looking axe as the first transform, and then a flail mace as the second.

Oathkeeper could turn into something like Ventus' Wingblade command style. Like it turns into wing-like attachments hovering behind Sora and he slashes quickly and glides along the ground. That would be cool.

Decisive Pumpkin could activate a limit where ghosts circle around you to do extra damage

good ideas people, keep em coming, if you dont mind

Three Wishes/Wishing Lamp could turn into a scimitar like Aladdin's own weapon. COuld have fire-based attacks and maybe a finishing move involving the genie?

The kingdom key slowly becomes the x-blade since its the blade that can turn into all others

ok heres some other ideas

divine rose, transforms into a rose with a long stem, rapid combos with thorns, finisher rains petal down

skull noise, first becomes a skateboard which has you ram into enemies, then becomes mr mew who flies around scratching, finisher mr mew turns huge and fires lasers from his eyes

Funny, I was going to make a thread about this, but someone else beat me. Well, since I'm here, let me share my ideas:  
  
+ Rumbling Rose: You get gauntlets with claws similar to Beast's, then you have a second transformation where you use rose's thorny whips.   
  
+ Hyperdrive: You get a couple of blasters similar to Stitch's ones. After that, you can combine them into a cannon/machine gun like the one from Stitch's red ship.   
  
+ Olympia: The Keyblade transform into a big hammer covered with lightnings, which Sora uses to smash enemies and inflict Electro damage. (Think like Thor's hammer).   
  
+ Mysterious Abyss: Sora uses two big shells as blades, similar to the Pokemon Dewott; a second transformation turns them into swords that also cause Water damage.   
  
+ Bond of Flame: Easy, the Keyblade turns into a couple of Chakrams just like the ones that Axel used in the Organization XIII. This allows you to use the same attacks as him.   
  
+ Hidden Dragon: Sora adquires an cannon similar to the one used by Mulan to cause the avalanche that defeated Shan-Yu's army. It can fire powerful fire balls, a strong flamethrower, and finish with a destructive fireworks' spectacle.   
  
+ Follow the wind: Similar to Monstropolis' yo-yos, Sora attacks with a couple of anchors with chains that gets bigger when they hit the enemy.   
  
I have more, but I will post them at other moment.

my hat off to your dewott reference 

The Dual Disc could turn into a pair of discs  resembling the ones used to store program information on The Grid. Sora could both hurl them as projectiles and use them as close range weapons similar to chakrams. 

The Circle of Life could create claw-like attachments on Sora's hands and feet (like the blizzard blades on the Crystal Snow keyblade) but maybe Sora would run on all fours similarly to Anti-form in Kingdom Hearts 2 and attack like an animal to mirror the lions in the Pride Lands. Attacks are thunder-based, perhaps?

Circle of Life: 'The Lion Guard' - Temporarily transforms Sora into his lion form outside the Pride Lands with 'Roar of the Elders' from Lion Guard as the finisher. 
In the Pride Lands, 'King's Legacy', a ghost lion form much like Mufasa's that behaves like Anti Form, entirely melee dependent but with increased speed and strength. How the keyblade creates this is that it transforms into pure energy and fuses with Sora. Same finisher. 

Pumpkinhead: 'Silver Kiss' (or possible 'Silver Night' or 'Silver Knight'), a Silver bladed gothic style sword inspired by The Black Death (see link below) based in poison, meaning it grants poison magic and moves such as Poison Slash. 
https://www.darksword-armory.com/medieval-weapon/medieval-swords/the-black-death-gothic-sword-1372/

Sweet Memories: 'Beekeeper', a large hive that behaves like a machine gun, shooting swarms of bees at the enemy. This also appropriately puts Sora in a beekeeper's suit. 

Counterpoint: 'Maestro', a conductor's baton that uses the same controls as either the Atlantica 2 singing or the KH3 carrousel attraction to conduct powerful booming sound waves in a similar behavior to the Pokemon attack Sonicboom while generating actual classical music. 

Just a handful that have come to mind. 

20 minutes ago, Ventus_ said:

Circle of Life: 'The Lion Guard' - Temporarily transforms Sora into his lion form outside the Pride Lands with 'Roar of the Elders' from Lion Guard as the finisher. 
In the Pride Lands, 'King's Legacy', a ghost lion form much like Mufasa's that behaves like Anti Form, entirely melee dependent but with increased speed and strength. How the keyblade creates this is that it transforms into pure energy and fuses with Sora. Same finisher. 

Pumpkinhead: 'Silver Kiss' (or possible 'Silver Night' or 'Silver Knight'), a Silver bladed gothic style sword inspired by The Black Death based in poison, meaning it grants poison magic and moves such as Poison Slash. 

Sweet Memories: 'Beekeeper', a large hive that behaves like a machine gun, shooting swarms of bees at the enemy. This also appropriately puts Sora in a beekeeper's suit. 

Counterpoint: 'Maestro', a conductor's baton that uses the same controls as either the Atlantica 2 singing or the KH3 carrousel attraction to conduct powerful booming sound waves in a similar behavior to the Pokemon attack Sonicboom while generating actual classical music. 

Just a handful that have come to mind. 

grand ideas, the black death themed one is bit grim mind you

4 minutes ago, ocean's rage said:

grand ideas, the black death themed one is bit grim mind you

*shrug*
I just Googled gothic swords since Pumpkinhead is a gothic themed keyblade and thought it looked right for the part. 

34 minutes ago, Ventus_ said:

*shrug*
I just Googled gothic swords since Pumpkinhead is a gothic themed keyblade and thought it looked right for the part. 

oh wait is there a sword called black death? i thought you meant the disease 

4 minutes ago, ocean's rage said:

oh wait is there a sword called black death? i thought you meant the disease 

Oh, no I did not. 
It is, in fact, the name of a sword. 
https://www.darksword-armory.com/medieval-weapon/medieval-swords/the-black-death-gothic-sword-1372/

I edited the original post to include the reference, I was in a bit of a rush at the time. 

11 hours ago, Ventus_ said:

Oh, no I did not. 
It is, in fact, the name of a sword. 
https://www.darksword-armory.com/medieval-weapon/medieval-swords/the-black-death-gothic-sword-1372/

I edited the original post to include the reference, I was in a bit of a rush at the time. 

bit of an easy misunderstanding on my part 

if they brought back fatal crest back what they could do is when go into the keyblade transformation it turn in the hammer form and then stage 2 is instead of a drill form a beast riding form ( close to bull riding ) where the player press the  action at certain times to prevent themselves from getting knocked of the beast form and ending the transformation early and the finisher is the beast self-destruct and damages nearby enemies .

ok heres another

bit obvious but have lady luck turn into cards, and then it could turn into one of the card soldier's weapons, finisher sora turns giant and strikes

Here's a few I can think of:

Photon Debugger: First transformation binds itself to Sora's arm, allowing him to lob energy balls at enemies. If you've seen the original Tron, this is based on the equipment for the Ring Game. Second transformation would allow Sora to access a similar keyboard to Tron's limit, allowing him to attack using data (think Tron's limit). Finisher would be a Recognizer, firing lasers in various lines.
Dual Disc: Solo transformation turns it into two Identity Discs. Sora can throw them at range or use them for quick melee attacks. Finisher would have Sora throw the Identity Discs in a circle around himself.
Oathkeeper: Solo transformation turns it into wings on Sora's back and two small daggers. Sora's able to glide around freely and attack quickly. Finisher would be a quick series of slashes that allow a blinding light to come through.

The last idea I have is for a Keyblads of mine, so bear with me.

Behold, the Lost Legend.
First transformation would be a greatsword. Heavier, slower strikes, but designed to deal more damage to one specific target, rather than Hyper Hammer's area attacks.
Second transformation would shift it into a sword and shield. These would be quicker attacks, also making Sora's guarding abilities better and he's much more difficult to stagger when blocking.
Finisher for this would be banging the sword on the shield to stun the enemies in front of Sora, before charging forward and knocking them away. This would require forward thought of positioning to get the best possible effect.

Two become one keyblade transforms into light and dark blades that grants combos similar to Roxas and gives light and dark spell in quick command. Shoot Lock is similar Sora/Riku limit "Session". Situation finish is Sora splitting into light and darkness attacking with with "Ultima" and "Stopza" spells.

On 3/26/2019 at 1:53 PM, Robaki said:

Two become one keyblade transforms into light and dark blades that grants combos similar to Roxas and gives light and dark spell in quick command. Shoot Lock is similar Sora/Riku limit "Session". Situation finish is Sora splitting into light and darkness attacking with with "Ultima" and "Stopza" spells.

Similar to my idea for it, with the light and dark blades. I'd suggest the Faith attack (Roxas in KH2, data Sora and roxas, and Sora in DDD have versions of this command. If you're not familiar, it's essentially pillars of light spreading out in a circle dealing massive light damage aoe and healing you) as a finish command instead and replacing Dodge roll and air slide with reversal.