Ongeki is an arcade rhythm game made by SEGA, which uses 6 square buttons, 2 side buttons, 2 start buttons and a lever as a controller.
The arcade cabinet uses Sanwa OBSA-60UK buttons with OBSA-LHS7F integrated lamp holders (100g switches) and 20g springs. These have common-anode RGB LEDs built in.
Currently there are a few people working on or have already made an Ongeki controller but there is no guide for one.
Things of note
You can find AI files for the top part of there controller including a low resolution top art here. For the rest of the controller there are currently no designs available so you would need to make your own.
The main 6 square buttons are 60mm buttons which are the same as for a Sound Voltex controller.
The start buttons are hard to find red and yellow colored buttons but generic 30mm buttons with a colored led under it also works fine.
The side buttons would need to be 3d printed as they are not on sale anywhere. For the button action you could have 2 iidx sized buttons or 2 sdvx sized buttons as the led, spring and microswitch holder. For iidx sized buttons you can find the needed STL files here and here. You would probably want heavy springs and switches in here so using Chinese buttons is actually recommended.
The lever would also need to be custom made, this could be 3d printed or welded. The general idea for the lever is that you have a rod connected to a pot meter. For adding resistance you could add bushings or torque limiters.
For the knob there is a STL file available for 3d printing here. It is made so you can put an M8 nut in there and then twist some threaded rod into it.
There are currently two different firmware available. They are both made with an Arduino Leonardo or Pro Micro and WS2812B LEDs in mind.
First one is made by Moldypie which you can find here.
The other one is made by Skogaby which you can find here.