I could imagine that the Metal Chocobo would give you a chocobo mount for a short time, not unlike a summon, that could heal you, make you fly, or shoot meteors across the battlefield. Its finisher would just be a chocobo rush or something, I dunno.
The lionheart would give you large lion claws that you could use for close-combat, with the finisher being a giant metal lion that would briefly rush a group of enemies and shoot fire.
The spellbinder would give Sora a lexicon not unlike Zexions that gave you a host of advanced magic, with the finisher trapping enemies in a book and banishing them. The keyblade itself would probably give you mp Hastega.
Jungle King would give you a slingshot and put you in a first-person-shooter mode where you fling projectiles at enemies. The slingshot would transform into a large vine whip that you could use to either whip enemies from far away or draw them in and finish them off with the whip transforming into a wooden sword. The finisher would have you map out a short distance where you would summon a vine slide and whip around your chosen path, smacking enemies like splash run.
Your lady luck idea is exactly what I was thinking, not gonna change it. Maybe the final move does a game of cards with an 80% chance of eradicating a tough enemy.
Oathkeeper will be cool: the blade transforms into a large pair of wings that grip onto your arms while the handle transforms into a giant bow and arrow. The bow, since it is attacking from a high point, will be in a first-person mode, and will shoot arrows of light that can either mark enemies for increased damage by party members or detonate after being planted. The finisher is an attack that summons 1,000 arrows in a small circle, cleansing that area of darkness.
Oblivion will be very different in approach. The oblivion will come in two different forms: the first formchange will have Sora coat himself in a black armor and become a heavy character. The jewel on the hilt of the keyblade will be on the forehead of his armor, and Sora will lose jumping while gaining increased defense, a greatly increased attack stat, and the ability to guard attacks and give huge reprisals by rushing enemies. The second form will have a great cinematic where darkness will seep from the armor and smash it, giving Sora less defense while severely increasing his attack stat. Since you don't gain oblivion until after Sora's sacrifice, Sora's transformation will greatly resemble anti-form, being the first sign of Sora struggling with darkness. Sora will also have a glowing red greatsword in this transformation that can slam enemies into the earth, as well as an anti-glide. His finisher will consist of Sora creating a burst of darkness in a ring, followed by him breaking out of his anti-form with a massive burst of light.
Whoo, that one was fun! Anyways, more keyblades!
One-winged angel will have Sora take on a unique form that gives him clothing resembling Shogunite armor. The keyblade's blade extends out to a flaming katana, which Sora can wield to deal fire damage from far across the field. The finisher is called one-winged angel and is similar to Sephiroth's most powerful attack, but not quite as devastating.
Diamond dust will have two forms. The first will give Sora icy gloves that he can use to shoot icicles at far-away enemies. The cool thing about this form is that Sora will increase the damage of his icicles if he uses an aero spell and he cannot use fire spells when in this form. The second form will give Sora a large icy flail that he can wield to smack enemies with mighty force. The finisher sends an icy storm across the field that freezes enemies.
Three Wishes will turn Sora's keyblade into a large scimitar that he can use to break enemy defenses. For example, the scimitar will either cut the coat off of a large body or fat bandit to break their defense or will cut the shield of the defender clear in half. Unfortunately, while he has this scimitar, his ability to jump will be removed, while the chance of him getting knocked into the air will also be nullified. The finisher will have Sora coat a small section of the field in a purple fog, confusing and trapping the enemies in it, with Sora slicing the fog five or six times to kill the enemies trapped.
I originally wanted to make the pumpkinhead a keyblade that gave you a power that would put enemies to sleep and give them nightmares, but then I realized that the weapon would become repetitive and practically useless against enemies. Maybe this one would also be a scary looking flail? And the finisher could be a bunch of ghosts swimming around and homing in on enemies?
Crabclaw would give you a diving ability like Ariel's KHIII summon. The keyblade would give you a mermaid tail and cover the field in a shallow ocean where you can dive and attack enemies. The finisher would transform the keyblade into a trident where you would coat the ocean in electricity, dealing massive damage to the enemies below.
Fairy harp would give you a massive magic boost and a pipe flute that would attack enemies, much like a bard from the final fantasy games. I think. Tbh, no good ideas about this one.
Olympia would give you giant hercules fists. Think God of War III's Nemian Gloves (or whatever they were called). The finisher would be like Hercules' finisher during his fight, where you create a large circle of light that damages enemies in its radius.
Divine Rose would give you two different forms. The first would be an elegant rapier with a red and green handle that would have a parry feature, allowing you to perform a finisher on an enemy by successfully parrying their blow. The second form would turn your rapier into an ornate, vibrant greatsword that drags on the ground as you walk with it, taking away your jump but coating any area you walk through in a thin layer of ensnaring vines. The finisher would trap various enemies around the field in vines and give them a death counter as an animation of a rose losing its petals appears in the right corner of your screen. When the last petal falls, these enemies will also perish.
I really don't like the wishing star, but I guess I can come up with something for it? The wishing star will give you the blue fairy's wings and turn your keyblade into a large slingshot, allowing you to rocket large boulders into the enemy from afar. The finisher will summon a giant wooden boat made out of light to crash into the field for area-of-effect damage.
And, finally, Ultima Weapon. The ultima weapon will give Sora a pair of golden blades and transform his clothing into a radiant, golden chainmail with a crown. Sora will be extremely agile in this form, with his attacks hitting like a truck. Sora will also be able to fly across the field, with his dodge roll being replaced with a teleport that brings him to the nearest enemy. Sora's finisher will have him summon a large crown underneath him that will be pierced with 1,000's of blades of light.