I've played with the idea of Kamen Rider as a character for a game based on Hypixel's Smash Heroes before, though in that case I was mainly thinking about Kamen Rider 1. I think a lot of the ideas in Smash Heroes really lead to getting the character to feel and fight "right". I've tried building my own custom movement and combat system and this is how I'd recommend doing it:
Make the durability of your transformation separate from your player health bar. Your "health bar" as a rider should not regenerate with food, be protected by normal armor, or be restored by normal healing potions. While not transformed, this secondary health bar should slowly regenerate. When your transformation's durability runs out, you get detransformed, knocked back, and take like 4 hearts of damage bypassing armor. There isn't a cooldown to prevent you from transforming again, but since the health bar doesn't reset you'll have barely any health in your rider form unless you back off and recharge.
To help a bit and make fights more cinematic and encourage varied strategies, changing forms while below half health or so should give a moderate health bonus. To prevent this from being abused, I would make it something like 1/2 your missing health the first switch, 1/4 the second, 1/8 the third, and none after that. This resets whenever you detransform and regenerate to full health.
Scale knockback with damage like Super Smash Bros! While at low health, you should be knocked back further. This actually helps you, since you can use the distance to avoid follow up attacks, change forms without being hit, navigate around your opponents, or follow up with a devastating ground slam or dive kick.
Movement and attacks feeling good is critical. I don't like extra keybinds for special moves, and you can't really do that with datapacks so in my experiments this is how I would do it:
Transforming gives buffed movement speed and jump height to begin with, on par with speed 2 and jump boost 2-ish for most riders.
Pretty much everybody gets a double jump for air control, which refreshes when touching the ground. Mobility focused forms get a triple jump or more.
Holding Sneak charges a super jump, with 2 modes:
-Jump while holding sneak to pounce forward in a big arc. Distance depends on how long you held: Leap an entire 14ish blocks up and 20ish blocks forward with full charge.
-Release Sneak, then jump right after to go straight up. Height depends on how long you held it: Tap it for a couple ticks for 4 or 5 blocks of jump, holding for 1.5ish seconds for the maximum height of 20 blocks.
Pressing sneak AGAIN in midair activates your flying kick, launching you forward in the direction you look and dealing heavy damage to anything you hit in a small radius, sending airborne enemies backward into the ground for massive damage.
UNLESS you are looking straight down, in which case it does a ground slam superhero landing thing, doing moderate damage in a wide radius below you.
Releasing Sneak ends the move prematurely, pressing it again after starting either move does nothing.
If you want to add a charged finisher version of the kick, it could be activated by double-pressing sneak (while having the ultimate move meter full or whatever).
While unarmed:
Your left click is a light attack similar to a normal punch: it is spam-able and deals very little knockback to let you keep comboing. In my version, landing hits with it would add extra damage and knockback to the heavy attack, which is reset when the heavy is used and decays over time.
Right click is a heavy attack with moderate sweep and knockaback, able to hit multiple clustered enemies. Has a cooldown like a 1.9+ sword swing. Could be buffed by punches, as above.
While sneaking on the ground, the heavy attack becomes a launcher move that sends enemies high into the air, although with a smaller radius. Note that sneaking starts charging a super jump, so you can use this to isolate an enemy and chase them in the sky OR to stun an enemy and leap past them while they're in the air and unable to chase you.
While in the air, the light punch makes both you and anything you hit hang in the air for a moment, so you can use it to stall your falls a little or exchange a fury of blows in a cinematic air clash. The stalling effect slows you even on a miss, but is almost a full stop on a hit. After the second miss, you lose the stalling power until you touch the ground.
To follow that up, the heavy attack in the air launches both targets backward, unless sneak is tapped at the same time in which case it spikes the target into the floor and leaves you in the air.
W could obviously balance the damage, range, speed, and knockback of the attacks depending on form, and use the weapon attacks instead of heavy punches, while Build, Geats, or Gotchard could need more elaborate special attacks but would work well with the halves of each form corresponding to the light and heavy attacks.
