I've got a few Riders of my own, some can be looked up and linked, but it's very long so I'd just reference them here and also introduce new things.
I've created a couple of Riders of my own, but I've only come up with a few stories. Here's a list of stories I've got:
Kamen Rider Gadez
Kamen Rider Voltur
Kamen Rider Ryuki: A Different Direction
Kamen Rider Unleashed
Each of these series are based on previous works with the exception of Gadez and Voltur to an extent, and I'll edit in some info as to how to get to them later, if people are interested. Some of them are just a click away on this thread, some are on my deviantart account (SSJ4Bardock).
Now, as for creating a Rider on spot... Here we go:
Kamen Rider Faiz: Stop Point
It's been 20 years since Kamen Rider Faiz (Lost Paradise Version) ended. Orphenochs and humans are beginning to live together, Smart Brain's influence had grown weaker and weaker as the days go by. People resisted, and it worked. The fighting for a better world, a better society, understanding of each other was coming to an end. They were almost free.
However, something happened that wasn't part of the calculations. The Orphenoch King, long thought to have been only of stories had awakened. He is now commanding an army of Orphenochs that are still willing to wipe out humanity. With nothing left to lose, a young generation of boys stood up to this offence. They broke into Smart Brain's last underground lab and stole what they thought could be hope: The Emperor's Belts. Built to protect the Orphenoch King especially, the Emperors Belt were designed to destroy any threat or nuisance that was out of order, however that would also mean that they'd have great ability to counter their own purpose... depending on the wearer. The young men came to two new belts, and one extremely old, yet functional belt. Having no choice, they took whatever they could and ran, however they didn't think that it would be a trap…
Almost a minute later, every door in the lab was locked. There was no getting out, someone had triggered the alarm. The boys were helpless, and soon the entire room was bombarded with Orphenochs, appearing from hidden passages all over the room, like from behind bookshelves and cabinets that could all be moved, to reveal a hole in the wall. It was a set up. Deciding on the moment, one of the boys puts on one of the newer belts, the Emperor Belt of Depths; the Mu Gear. The belt slipped onto the boy's waist easily, and pulling out the new touch Phone, he flipped it open and pressed the activation code: 124. He then placed the device into the belt and announced the transformation, with dark blue photons firing up from the belt, the boy was changed into a warrior clad in dark armour with shinning green eyes. This was Kamen Rider Mu. The young Rider began to fight for his own, but he was still a rookie, and was knocked back into his original form, the Mu Gear not being of help.
Just as all hope seemed lost, an Orphenoch appeared, attacking the ones targeting the boys. It looked like a wolf, but it moved too fast to be too clear. Before anyone saw anything, the mysterious Orphenoch grabbed the old and dusty gear and puts it on, entering the code of 555. Placing the phone in the driver, red plasma rays fired from it, transforming him into the one and only legendary saviour: Kamen Rider Faiz.
Faiz fought with the enemies, and destroyed all of them, guaranteeing everyones safety. As the dusts cleared, he turned around and looked at the boys. Grabbing the Mu gear, and the other gear that they stole, he handed it to the boys and said "Protect your dreams… and the dreams of others."
The boys accepted the belts, and as soon as they did, a sound beeped from the belt and it announced "Deformation". Faiz's armour lit on blue fire, and soon he was turned to dust, and even while it happened, he didn't even flinch. The boys panicked and watched helplessly as the future is rested upon them. However, they weren't going to be alone. In the pile of ashes, laid the Faiz memory chip…