Hi there!
Here are a few suggestions of mine:
1) Add a 'stabilizer' feature to smooth the strokes, like the one in PaintTool SAI. It slows down the stroke you're drawing, making it more smooth. The amount of slowdown depends on the number set. Here's a
video I found, where the artist uses this feature at maximum amount to draw line-art for his character. I think that would be a nice addition for Black Ink too.
2) Possibility to use image files for tracing them (not only taking color from), that would be good for inking sketches too. Though maybe when layers will be present it wouldn't be needed.
3) Change the rendering engine's antialiasing algorithm a bit.
Here I made two lines. After I make undo (2nd pic), appearance of strokes slightly changes, becomes kinda blurred or 'antialiased'.
Don't know about other users, but it kinda disturbs me, because I become unsure of what the drawing will look like in result. And personally I don't like 'blurred' lines.
4) Make possible the splattering of strokes. I've experimented a bit but couldn't reach effect that when I draw with high speed ink would go even further than where I draw, so it would splatter a bit. Scattering is not the same, it only drops shapes. Can't really say if I saw that in any other 'big' programs though to make a more clear example... Well, that's unless I'm missing something in Black Ink.
Maybe I'll think of some other suggestions later...