making type 6 a cylinder or sphere or something

That does not make sense in my eyes. For those shapes we have already our primitives. And defining such meshes like you suppose, what is the benefit? We also need to fit these meshes with connecting shapes. In my eyes we do not need any new shape indicator as in the 1-5 lines all can be expressed.

But the idea of Steffen sounds good to me. One question that needs to be answered is "Are files with lines 6 (or above) readable by current tools?".

Edit:
MLCad - "Error in line, should file be loaded" and the line is deleted if you proceed and save.
LDView - "Error parsing line" no effect on visual result
DATHeader - "Error using 0 comment" - strange error message. Correction of that line do not work, so it stays :)