I agree this is a good suggestion.
Currently default line width is 0.00 mm. This often leads to dispaired newbies wondering why there is no or very faint print out.
I have opened an issue at github and suggested to change it to 0.25 mm and also mentioned that in more advanced CAD apps it is possible to set the default line width within Application Preferences.
New developer melwyncarlo has dedicated some effort into it, but I don't know how the solution looks in detail, we have to wait for it to be integrated.
You may open an issue at github yourself with the explicit request to implement a setting possibility in Application Preferences.