I have spent the last 24 hours on ThinkerCad. I find the software good, but too basic. Several things are missing, I suggest improving:
1- Add layers option. In order to be able to work on the sub-layers without having to move everything.
2- Add grouping options, such as substraction, exclusion ...
3- Add easily manipulated x, y, z coordinates.
4- Give the option of modifying whatever you want.
5- Have a mesh option, in order to be able to micro-modify a shape.
Are you thinking of making a more complete version? It would be very nice.