Alrighty, so I thought of a way to make this with no plugins and only command blocks. I dunno how reliable it'd be on a large public server, but here's how it could work:
The first picture is your DDR controller. Just like the real game, you step on the button that you're being prompted to step on. The next couple of pictures are my (very ugly) rhythm screen. At the top is a falling sand spawner. Each time it spawns, there's a 25% chance for the block to be blue, purple, red, or green. The falling sand block falls in the same row every time.
The following is very complicated redstone that I'm not even 100% sure how to make; you've been warned.
The falling blocks land on tripwire hooks, sending a signal to a redstone torch. Each torch is connected to the corresponding color's pressure plate, the ones that you step on. If you step on the pressure plate at the same time that the falling sand is inverting the redstone torch, the signal will be carried out to a command block which adds 1 point to your scoreboard. (I'm not the best at redstone, so I don't know how exactly I'd wire this up. It'd be a lot of trial and error and I am way too lazy. I'd rather leave that to someone who's an expert with redstone.)
I'm assuming there'd be 8-16 players per game. I set my block spawner to spawn 1 block every 20 ticks (1 block/sec). That means that there will be 16-32 entities per server at a time (entities including players.) To compare, my sheep farm on Skyblock has 6 sheep per every color of sheep (6 sheepx16 colors=96, 3-6x more entities than this entire game) and I experience absolutely no lag when shearing sheep. Entities and spawners actually don't take up a lot of resources on servers; the majority of RAM is dedicated to loading chunks. If the owners/devs set the server render to 3-6 chunks, this number of entities and chunks would be less than my single island on the Skyblock server. The game suggested is very cheap on resources, which is probably its most attractive feature.
