I suspect you want people to be able to extend these APIs without knowing abstract algebra and category theory.

No, I definitely do not want people to extend any classes if they do not want what they are doing. This is a Java approach and we want to move away from it. I want people to use existing constructs and possibly write extension functions.

Yeah, I think it makes sense to just provide a small, lightweight core algebra. You could easily spend a lot of time writing abstractions for more complicated things

Indeed. I will open a KEEP-like repo later today for discussions (it is still morning here).