What sorta ideas did you have? I was thinking how tricky it would be if you cut off rightward rotations or maybe scrambling the die after every correct answer could be interesting ways of adding difficulty without breaking the simplicity of the game.
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Interesting ideas, but I don't think these would fit in very well. It's very important that the player only need to rotate once to match each new target number and taking away any control would ruin it, also the flow is no less important, since the countdown gets quite fast, and scrambling the die would only make the player disoriented and break the flow. To be frank, it looks like the core difficulty of having to match the number quickly is already high enough, judging from other comments and high-scores. Tuning the speed a bit better is what I would do at best here.
What I was actually trying to do during development is to make the target numbers appear randomly from all six sides. The problem with that is it introduces a third axis of rotation, which becomes confusing very quickly with the current type of input. The only way to solve it I saw, was rotating the die through mouse/touch input, but implementing it properly would require a lot of time, so I made a decision to go back to the simpler form, so I could finish until the end of the jam.