I liked NaroCAD project. It is flexible but have some misses. I tried to make some changes in it's code and I've start with simpler things.
I had working to add a line and a circle tool in sketcher. The line was pretty easy because I've started from the rectangle. Anywhere I've see the word: rectangle, I put line and adapting the code. The things go easier as much as I've did that.
I continue with circle. The circle seems harder so I did it as much as I could and at the end I went blocked. But seems fixed from the main developer of Naro (btrx).
Another small touches were: a splash screen which was fun to be done and I hopethat I will add from time to time things like that :)
I think that some things can be done, mostly on UI side like making an toolbar editor. If you like the CAD coding and you know a bit of OpenCascade there are a lot of places you can help. I will try to add in the third iteration the middle point highlight and touching the UI bits.