Implement a “visual debugging” feature to help users investigate hidden physics issues. Should record collision shapes, contacts, forces, custom events, etc. over time. Viewport overlay to show such data, make it selectable for reading numeric details. I would consider a feature like this essential for both users and developers. Quite a bit of work, but not so difficult to design (many game/physics engines have some variation of this feature).
This has been worked on during the previous GSoC:
I don’t think it would be too much work to do some cleanup and integrate it into master.
However I haven’t had time to do it, so if you feel like you want to have a go at it I would be really happy.