As cliched (and HP expensive) as it sounds, I'd want to add a Maths in a smaller system like this. Will open up a lot of possibilities and can be a standin module for a lot of functions while you learn the basics, also does envelopes very well imho. You have limited space, and it is a big module, but I keep being surprised at how much I enjoy working with it and how much I learn from it.
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