Heres an idea for a fic I had.
A god-like interdimensional being only known as the Game Master decided to use the Equestria Girls universe as one of his arenas for his Mega-tournament forcing many of the contestants from alternate MLP timelines and series into his tournament granting them an henshin device. Two of them being Human Sunset and her father, Solaire Shimmer. The last person or team standing(whether by death, surrender or destruction of henshin device) gets one unrestricted wish. Human Sunset has terminal cancer and it's being held at bay by the Game Master for the duration of the tournament, and her father is fighting so he could use the wish to cure her. He lost his wife, he won't lose his daughter. Solaire Shimmer is a former thug who straightens out his life when his daughter was born. Pony Sunset Shimmer is dragged into this tournament with her alternate personas as partners.

List of characters/teams(work in progress):
Team Shimmer
Sunset Shimmer(pony) aka Kamen rider Inferno
Henshin device: Tamashi driver
Partners:
Daydream Shimmer: Allows Sunset to become Inferno Guardian which prioritize defense and endurance.
Sundown Shimmer: allows Sunset to become Kamen Rider Inferno Wrath which prioritizes attack and speed.

Solaire Shimmer/Kamen Rider Rogue.

Sunset Shimmer(human) aka Kamen Rider Blaze Rogue
Henshin device: Transsteam gun
Partners:
Daydream Shimmer: Allows Sunset to become Blaze Support which prioritizes defense and endurance.
Sundown Shimmer: allows Sunset to become Kamen Rider Blaze Assault which prioritizes attack and speed.

Team: Invincible Knights
Flash Sentry aka Kamen Rider IXA

Ringo aka Kamen Rider Brave

Brawley Beats aka Kamen Rider Baron

Team Masamune:

Starlight Glimmer(pony) aka Kamen rider Xelor
Henshin device: Gamer driver with Xelor Quest Gashat

Dan Masamune aka Kamen Rider Cronus
Henshin device: Buggle driver MK 2 with Kamen Rider Chronicle gashat

Spitfire(pony) aka Kamen Rider Areo
Henshin device: Gamer driver with Jet combat Gashat

You might as well change Solaire to Kamen Rider Odin since his shtick is basically the same as it was in Ryuki. Minus the father/daughter bit. But that's it.

6643001
...Nani the fuck?

So this Kamen Rider Ryuki + Chou Superhero Taisen... This does Not bode well...

6643376
+ a holy grail war from the Fate series

6643780
The only real difference between the Rider War and the holy grail war is that the later has a backstory and actually makes sense, as opposed to the Rider War in Ryuki, seriously, where does the wish even come from in that one.

6643799
The wish was your own suffering.

6643802
The only one that was suffering in that series was the viewer, seriously, Ryuki was torture to get through.