Had an idea for a rhythm game keyboard, would it even work?

That would work a lot better. If it worked for Nintendo on the NES, why shouldn't it be able to work for me?

Actually, you're right! Instead of this polling protocoll based on device address, you could simplify it even more, and implement it like the SNES. Except instead of using a shift register, the devices will wait for a configured amount of clock cycles before they assert the data bus.

The only thing you'll have to think about then is how to assert a RESET signal via just two wires. While possible, it requires some thinking. That way you could technically hook up infinite cubes, not just 8 (assuming the cubes themselves could support anything higher than that).

You guys are making things unnecessarily complicated. Just buy a teensy board on the cheap, load it up with a game pad or a keyboard preset, solder some switches to the corresponding pins and you've got yourself your dream rhythm game keyboard. If you want to go for the magnet connector thing, all you need for 8 switches is a ring of 8 signal pins with a central ground pin on each side of your cubes. It's a little bit esoteric without a schematic, but consider the central Teensy to be your microcontroller with 8 dedicated pins numbered 0-7. Each pin on the faces corresponds to one of those pins, and each cube is numbered. Each individual pin is connected to the rest of the pins with the same number, so they function as a pass-through. That way, regardless of where you put Cube 0, it will always be connected to pin 0. I've actually seen kits that do as much, but it's so trivial to make that it's be a waste to spend money on it. If you need a diagram, I could probably whip one up for ya.

Yeah, the reason I've been stretching this protocoll nonsense is that I don't think there are pogo connectors small enough to fit next to eachother in an 8 configuration into a keycap-sized cube. And then you'd need 288 pogo pins (9cubes * 8pins * 4sides) if you want to support all sides connecting to all sides.

Edit: MagSafe can only fit 5, and let's not even talk about the physical durability of those 8pins at that small size, even if you could fit it next to eachother.

The size is somewhat constraining, but not a deal breaker by any stretch of the imagination. It's also less connections than you think since all the same-digit pins are bridged. If you want to overcomplicate this build, you can have a microcontroller in every key cap and control them as an array of individual buttons with the central cube working as your hub, or you can use clever timings. This would increase your input lag, which is undesirable in a rhythm game controller, but chances are that the delay would be imperceptible to a human. I would keep it bog standard analog and place the relevant connectors on the underside of your cubes, have you considered that? In fact, you could utilise a "playing mat" that the buttons slot into wherever they're needed, with the remainder of the space covered by flatter dummy cubes - that makes the whole thing even more trivial. It's just an 8x8 grid.

yEa i did have that idea

The absolute easiest way to do this is to create 3x3 pin arrays on the bottom of your key caps with matching recepticles on a PCB, or even a perf board, bridge connections accordingly and grab a small microcontroller to control the show. It doesn't even have to be a teensy (they're just really convenient), there's a lot of ATtiny alternatives that can connect directly to USB, some about the size of a keyboard key.