There seems to be a great interest around the Shape Builder Tool… and I am also one of the very interested, in fact things like that were just in my horizon before I found these requests here. As there is just something being implemented, I decided to hurry and join my “two cents” (I hope this expression is here well used).
First some related (I think) features:
1 – The knife tool is too poor, I think. Please, let us design a path along which the slice would be done.
2 – Definitively a Paint Bucket Tool (a jerry can that fills an area with a color/texture/gradient) is very practical, never mind all the work-arounds we can imagine. I know it seems too simplistic and non-minimalistic… or perhaps not.
3 – Examples:
Look at the first image: a rectangle and a second shape (a line) on it.
Now, to make the second image, I had to draw a second shape with the Pen Tool (I tried, hastily, to make the curved line similar to the first one, changed the color fill and superimposed this second shape to the first one) I could have been more precise, of course, and perhaps there are better workarounds, but it is unquestionable that a Paint Bucket Tool, would make this all immediate.
To make the third image, I first used the Knife Toll with a rectilinear cut on the rectangle shape, then I edited both shapes individually, trying to make both curved lines look similar.
The fourth image was done like the third one, but at the end I wanted to paint an inside and an outside border to the shapes, instead of filling something with a color and added a second border to the shapes.
Lots of work to achieve simple things.