I've been thinking of experimenting with building a Sketchpad-like system on it. With Rust, you have access to LLVM fairly easily, so you could probably do something with JIT compiling code (maybe a Smalltalk dialect) to give whatever you build a programmable interface.
Why Smalltalk? It's simple syntax would be easier to deal with on the reMarkable, and you might be able to look at doing handwriting recognition to let you write the code with the stylus. Could also go with a Lisp, for similar reasons.
All of this is just speculation because I haven't started it for lack of time, but I'm really interested in exploring what's possible in that space. The system specs will probably be constraining (I haven't looked at them recently) but I'm sure they're far far more than what Sketchpad had.