I went with keeping the default paint in my custom nodes simple.
Why?
The benefit of having a simple default paint is that you don’t have to deal with merge problems with paints later. As an example, I never had to deal merge issues with vvvv’s default paint (white fill). That is why I have decided that my nodes would either have unmodified Fill or unmodified Stroke as the default paint.
Another benefit is the user would be already aware of what the output of unmodified Fill or unmodified Stroke looks like.
If the user encounters an issue like this one where only having a stroke make sense, I’d put this in my node definition, in between Paint input pin and the shape I want to pass the paint to:
This would ensure the shape always appears with a stroke.