RGB LEDs can assume one of 7 colors (assuming that the anodes are either fully on or off, and not counting "black", which is just all of them off). To date, Симон only uses 4 of them - red, green, blue and yellow. Another way to screw with the player is to change the game's palette at the start of every game to pick four out of those seven. So a game might be in cyan, yellow, magenta and white for example.
This might be going a step too far. I'm undecided. But at least on a hardware level it's certainly possible.
That said, I don't want to turn Симон into a 5+ button game. Going to 5 buttons would be doable without major changes, but 6 or more would require putting the buttons in a matrix and it just gets a lot more complicated and begins to stop honoring the original legacy of Simon (which is a nice way of saying "stealing the look and feel").